Only later into production, originally this was going to be coming right off the bayverse, before they went ahead and soft-rebooted the whole thing. That also doesn't change the fact that it was designed to be a Bumblebee-centric spinoff, he's easily the most marketable character in the entire franchise so it was gonna happen sooner or later I suppose. But again, this is already after tons of other Transformers movies, which is an established brand in film now.
Just because it only has his name on the title, doesn't mean it only has to include him as a character for crying out loud. This isn't the MCU, we can just have our core cast from day one without any of the bullcrap.
Maybe it's just me, but at this point I feel like Tails and Knuckles are just as much a part of the brand as Sonic is: they're a recognisable, colour-coded trio with distinct personalities. You literally already have a hero cast served up to you on a plate. Use it. At least Sonic and Tails, for crying out loud, they're one of the most iconic duos in gaming.