Sonic self-inserters want to be as cool as Sonic is.
Tails self-inserters relate to Tails' own aspiration to be as cool as Sonic.

Thus, both Sonic & Tails have fans who are likely to "self-insert" as either of the two for either of these reasons. And this makes perfect sense because Sonic & Tails are basically two very specific archetypes of protagonists that are exceptionally common throughout all media everywhere.