Why only have the link to bitcoin core at the top, and not classic, unlimited and XT as well?
Because those are altcoins, and this forum is called
Bitcoin Talk.
No, these are called options.
The entire point of bitcoin is that the USERS can choose. Not any one group of devs.
The Core devs are obviously trying to centralize bitcoin under their rule, and not giving the other dev teams a fair chance and an equal playing ground.
This is centralization
Classic, unlimited and XT are all bitcoin, and I'm fairly sure they would all be more popular than Core if they would have the same opportunity to be listed as 'official' bitcoin clients.
Core is in no way more 'official' than Classic, Unlimited or XT.
Technically, until a Bitcoin fork obtains consensus, it is an alt chain. Core is "Bitcoin" at the moment. Should Classic, Unlimited, XT, obtain consensus to take over as "bitcoin" then the forum would be about that chain.