Twitter and Discord are both companies with operations in the US and EU and are therefore accountable and registered. A number of the servers on the list you provided are ran by private individuals or organisations that are registered ‘off the books’ (in obscure third countries with limited transparency requirements).
Having to track and administer what is, effectively and literally, several different websites that are operated differently and independently induces more costs than it provides benefits. As another poster mentioned, this would also complicate PolicyService, as all instances have their own administration, governance and content policies, and therefore would have different regulatory requirements.
There are also many other platforms that deserve support over Mastodon, and would foster better communities for developers, such as Instagram and TikTok, as these are used more by players. To put it crudely, excluding genuine social interaction (i.e. with already known people), Mastodon is really just used by those who are not welcome on mainstream social media platforms (such as extremists and proponents of objectionable content) or those who are technical enough to care about federation (who are unlikely to care for or use Roblox social links and make up a very small percentage of communities on Roblox); this is representative by the platform’s general lack of internal discovery.