So, why were the fleets chosen again?

Well, I wasn't around in beta so I'm just going to have to assume the beta testers had good reason for recommending BW put the entire population in grey, lifeless homogenous fleets.

1) The "main social hub" is whatever the community makes of it. The fleet is definitely slightly more convenient than either Coruscant or Dromund Kaas, most of the regularly used features found on the fleet are actually available on those two as well.
2) I'm certainly GLAD myself that the community decided to make the fleets the social hubs. Every tried jumping pipelines to get some datacron on a laggy server? Coruscant is already bad enough for that, but gladly it's still doable outside peak hours. If Coruscant turned into a main hub, it'd become nearly impossible to make some of those jumps, especially the track in The Works would be a major pain then, with half your jumps registering late or simply not registering at all.

I hope they change it so we can have Coruscant and Dromund Kaas as main hub for all players, PLEASE do it BW!

Actually, the ultimate solution would be to add the same PVE/PVP gear vendors, as well as guild banks and all the other fleet stuff to capital worlds, so that people have a CHOICE. However, I can only see this happening with a global chat channel.

Actually it wouldnt need to be completely global chat channel. It could be a chat channel between the fleet and a certain area of the capital world where the vendors are. You could even have a direct teleporting system between them (like orgrimmar and stormwind, but quicker). And to remove annoying load times when moving between them, that area could actually be a part of the fleet, so that when a player from another area of the capital world comes to the hub area, he would have the loading screen.

To make players move between them more, not all the important vendors would need to be in the same hub. And if one day they introduce swoop racing and pazaak, players would move between them even more.

I was not an early beta tester, but I seem to recall beta testers saying that the game used to have all the key hub inducing stuff (like GTN, terminals for ops, etc.) on every planet and that they late in beta changed this. Personally, I think forcing a central hub somewhere in the game is good as it brings people to a central gathering point. Since planets are all pretty much level driven, and players of all levels (after the starter planets) would need access to centralized content it makes sense that they made the hub a fleet that is not part of any planet. You are portaling everywhere in this game anyway, so I really don't see what the issue is that some people have with Biowares decisions here.

changing the central of the game to another place is great cause players feel something new happened but not spending another whole freaking year on the same Space station!

Yeah, some people like it when an MMO does this. Yet if we take the gold standard some people declare for MMOs (WoW) the developers decided it was bad in the long run and went back and repurposed the original capital cities as hubs.