Either only Mastodon instances will be suggested because "Mastodon is the standard"/"Mastodon is the default"/"The Fediverse is Mastodon"/whatever, maybe actually because whoever designs something like this doesn't know that the Fediverse is more than Mastodon.
Or you get some more or less hidden setting that allows you to choose an instance of another project. If you don't, you only get Mastodon. Result: still 99.99% Mastodon instances being chosen.
What should actually happen is all projects being treated equally, at least all with the same purpose. If there's an Akkoma instance or a CalcKey instance that fits you, then it should be suggested. And if someone demands quotes or text formatting, then they shouldn't get Mastodon slammed into their faces over and over and over again as if there was nothing else.
Or if someone is looking for a replacement for Facebook or Instagram, they shouldn't only get Mastodon instances to pick. The Instagram replacement in the Fediverse is Pixelfed. And the number one Facebook replacement is Friendica, and it has been that since 2010.