I personally believe that SEGA has always liked the cast and wanted to make use of them. Or at least, SEGA of Japan always wanted to make use of the non-main characters, what with Sonic Channel having always featured artwork and comics of the whole cast (including the Babylon Rogues and Marine), and the wholly Japanese-made Sonic Runners bringing back Mephiles of all characters. Not sure about SEGA of America, but I can assume that the only characters they didn't like to use were Big the Cat and Cream the Rabbit, strictly due to their respective personalities/archetypes having more Japanese appeal (though Big certainly turned that around by now).
But of course, 2006 saw almost the entire Sonic fandom and even the gaming community in the West start calling the characters "Sonic's shitty friends" and blaming them for everything going wrong with the franchise at the time. Both branches of SEGA had to give the cast a break after that, and no doubt the use of the characters was further discouraged with fans making artwork of Classic Sonic being horrified with having to associate with his future friends, and certain reviewers docking points off of Generations for daring to include the cast. Like, you can't get a clearer message about how undesired the characters were than that nonsense. So, the main series games would only focus on Sonic, Tails, and Eggman while the rest of the cast could only be included in multiplayer spin-offs, and only because multiplayer spin-offs require additional characters to fill out the roster.
Since then, though, attitudes towards the extended cast began to improve due to varying factors. Unleashed and Lost World effectively silenced the fans and critics who blamed the friends for "alternate gameplay" since both games were strictly Sonic only, with only two/three other characters involved in the plot (but not even by much), yet ended up having differing gameplay anyway. And indeed, the fact that the franchise as a whole wasn't actually improving with the complete focus on Sonic, Tails, and Eggman further proved that the extended cast were never really "everything wrong" with the franchise. Fans who complained about the cast simply because they preferred the Classic Era with its minimalistic cast were appeased with Mania. And of course, the fans who were children in the 2000's grew up and became the dominant voice in the fandom; bringing up how much they love and miss the characters that used to be major players in the franchise, and pointing out those characters' merits that were previously ignored. It also helps that the type of gaming journalists who contributed the most to the "Sonic's shitty friends" shit aren't taken seriously anymore, with stuff like "Too much water", "Sonic was never good", and the "Cuphead tutorial level" ruining much of their credibility.
The hatred for the whole Sonic cast is practically dead and current fans are begging for them to return, so SEGA is more than happy to oblige. Especially SEGA of Japan for the reason I said in the first paragraph.
So, to day is the first day of Autumn...
And the day Sonic CD first released in Japan, apparently.
Happy Autumn, everyone, and happy birthday, Sonic CD!
Happy birthday to Amy Rose and Metal Sonic, too, who first appeared in Sonic CD.