I wasn't involved in either close voting process (both of the referenced questions are closed now), but I'll try to outline possible reasons that might give you some ideas about what might have happened.
The word "perfect"
The first thing to note is that you used the title "What does an ideologically perfect solipsistic government look like?", which implies that there is a perfect state of being solipsistic. Looking through your question and the comments it looks to me like your understanding and intention of that phrase was to imply "a society where everyone adheres strictly to the principles of solipsism".
But the word "perfect" doesn't have this meaning in a strict sense. This might seem like being extremely pedantic - and quite often it is extremely pedantic - but we have many problems with questions that simply assume everyone has the same understanding of what the "perfect" state of something is. Take for example a question about the "perfect weapon design". Valid questions are: What situation are you in? Is the "perfect weapon" one that never runs out of ammo? One that fires faster than any other weapon? One that fires projectiles with higher mass? One that only incapacitates, but doesn't kill the target? There are many possibilities and defining what you think is "perfect" is important.
Oversimplified: the word "perfect" is for many people a trigger phrase similar to "I am looking for ideas" or "opinions welcome" - each of which invite votes to close, most often about being "Primarily Opininion-Based" or "Too Broad".
The other question simply asked what the consequences are. It's too broad - and it was closed as such - but it doesn't have the same problem of defining what "perfect" is. And as "perfect" is up to you - and only you - your question received votes as "Primarily Opinion-Based".
It also looks like there is an already-defined model of how such an "ideologically perfect" government would work, which made people assume that experts on other sites would be able to tell you how it would look like - they concluded that it's off-topic here and might be on-topic somewhere with more domain knowledge. It looked like you are discussing some already existing model, which is also why people asked about where to look up that already existing model to be able to understand the goal you want to accomplish. In the eyes of close voters it might have looked like the other question simply stated a starting point and explicitly wondered what this might lead to, while yours defined that an ending point and wondered what the implications are on a different part of the society - without mentioning how that ending point looks like.
Short texts are often not so well-received because they look like it's more of a "shower thought". Some quick idea that you want to get feedback on and use as the starting point. Quite often short questions and answers miss important things that need to be considered for good questions/answers. Not always, but often. This is more of a sentiment thing that might explain some of the reactions being more favourable for the longer question than the shorter one. Showing what problems you encountered while researching the base problem can be a good way to show effort and steer answers in a certain direction.
Missing criteria for defining better answers
A question is primarily opinion-based if every somewhat reasonable answer is exactly equally valid.
You stated in one of your comments:
I haven't seen very many questions where the author explicitly stated criteria for judging best answer.
In that case these questions are likely closed or likely to be closed at some point - especially popular questions can go quite a while without being closed or with multiple close/reopen cycles. But without any criteria a question is primarily opinion-based. This is one of the most discussed close reasons on the site and regularly something falls through the cracks, so you might very well have encountered something that is open and primarily opinion-based, especially when looking at very old questions, but that is not what the current guidelines say. You need criteria.
The fact that you stated this shows that you didn't give it any thought, which may very well be cause for a close vote as "Primarily Opinion-Based".
Just to be clear: the criteria don't have to define a best answer and it's okay if many answers are valid. But there have to be criteria to somewhat objectively define which answers are better than others.
It's quite true that people should know the term when answering your question. We can't accept that people without any knowledge of physics answer orbital-mechanics questions that are asking for formulae for example, so we also shouldn't allow people to ignore philosophical terms. And it's not your responsibility to define each and every thing in your question.
But we are not a site of specific domain knowledge experts. We have many people with a broad area of "expertise", so it's always a nice thing to for example link to Wikipedia for terms that the average user on the site might not know. It's quite possibly that someone will use this as a starting point to write an answer even if they haven't heard the term before they encountered your question. It also shows research effort and makes it easier to get to a common understanding in cases where there are differing definitions of something. You are supposed to research a topic and then state at which point you had problems. The StackExchange Q&A format is not a starting point for questions, but an ending point and you should show which steps you were able to go alone and where you encountered something you couldn't solve yourself.
All in all it's simply a nice thing to do to indicate what you have done and help others that think your question is interesting understand it. Being nice helps people who you expect to work for you for free by answering your question feel they should be nice to you in return.
People on this site are experts about building fictional worlds, not experts about discussing philosophical concepts.
If you would like to check your wording or get feedback on a draft you have in mind you can check out our Sandbox on meta, which can help a bit in reducing the chances that you will encounter such problems.
For referencing material I recommend that you think about whether the average user will understand it. If you think that's too abstract it might help to ask yourself "Would my average family member understand this word like this?" (assuming your family does not consist solely of philosophers/rocket-engineers/...) and if the answer is "No" you should add a link to Wikipedia and spend half a sentence to explain how you are using the word in the context of your question.
All in all both of the linked questions are too broad/ primarily opinion-based and both are closed as such. It took a different amount of time because of some differences in the presentation, communication and specific topic, but the rules are being applied consistently.