Chaos control
The closure reason likely comes from chaos that happen during a first contact and establishing a relationship between humanity and aliens. Indeed, the smallest change in starting conditions can vastly change the outcome. These conditions can be as little as the current leader's ideal or in which order aliens met them (meeting the USA's president today before Russia's or vice-versa would cause some issues, for instance). It's not taking into account a single leader can take charge of extraterrestrial relationships if countries joined together after this world-disruptive event, a la United Nations but not like the United Nations (that's a possibility).
Besides, most stories involving a first contact happens to have very wild reactions from those pesky humans, going from intergalactic federation to all-out war passing by some weird linguistic shenanigans, and all are likeable (to the viewer at least) and believable in their context. This makes the task of finding "a more likely" answer much harder to make, since most choices are already made believable.
What next?
I'm not sure I would have it closed, first because I'm more fond of reopening topics now. However, I'm not inclined to reopen as it is now either because of the aforementionned chaos above, which makes the strategic and probabilistic a.k.a "most likely" approach quite harduous to do.
I cannot guarantee its reopening, it's a tough one. The classic way of going around this is to think of an approach, and ask if this one would reasonably work given the context (reality-check) instead on how you could make it work. You have to do some work by yourself, but you can then cross ideas that didn't go out as planned and use and overuse the ones that went smoothly like a smoothie.
Another way... Countries have very different culture and approaches to militarization and things that looks like colonialization (especially in old colonies vs old colonial empires). So one thing that might help is to focus on a country or two at most, telling the others "already agreed". It'll reduce the variables, and therefore the chaos triggered by those. To sum up this idea, dust off one of them to see where you roughly need to go for the others.
In any case, if you make a new question around the same topic, I'd advise to go in the sandbox before posting on the main site. It'll allows some people to review it before and reduces the risk of being closed. Like I said it's a tough one and I believe some help can be appreciated.
Alternatively, you can fully embrace it's too situational to ask what is more likely and think about what you wish to tell and... just make the events go that way. After all, if others managed to tell such a wide variety of alien encounters and colonization attempts, then so can you :)!
