Do you have any theories on why Sonic Mania never got Steam Workshop support?
Limited time and limited budget.
Like, when Sega enabled Steam Workshop support for their Genesis Emulator, that's because there were decades of user tools already out there in the wild, with hundreds or even thousands of ROM hacks ready to be dropped in to that system.
If Sonic Mania launched with Steam Workshop support, we'd largely be working with the same tools used to create the game, which may not be very user-accessible. Making a Sonic Mania level probably isn't as simple as Super Mario Maker, and probably not even SonEd2. It would only support exactly as many features as the developers themselves needed, and only in exactly the ways they'd need them.
The argument could be made that friendlier user-made tools could come eventually, and that would be true, but it also comes down to what Sega would be comfortable with, too. Official development tools would require QA (quality assurance) testing before release, which costs money, and if you figure the tools were scrapped together to support only the bare minimum of features required to ship the game, then the code would most likely be a patchwork of different messes. Which means a security risk. Which means it wouldn't pass QA.
Steam Workshop support would undoubtedly add weeks or even months to the development timeline as they plugged all the holes in their tools in order to prevent people from accidentally damaging their system.
That's a lot of money and a lot of work for something that probably wouldn't even be that useful anyway. It might be smarter to just start over from scratch and rebuild brand new user-facing tools, keeping in mind cleanliness and accessibility. But then that's even more months of development. I mean, just look at how long its taken for Sonic Studio. That's been in development for, what, three years? Now that's just one guy, and it's something he only works on in his spare time along side other hobbies, but still. It's not something that's simple, easy or fast.
And that's not even touching on the backend support of actually plugging in to the Steam Workshop so that you could upload levels and then download and put them in to the game. That's a whole separate tech of menu systems and protocols and more QA.
Plus, for as cool as Workshop support was for their Genesis Emulator, it’s become an unmoderated cesspool of people uploading pirated material, something I just know is probably going to get Sega in to trouble eventually. Sega doesn't actually care about the Steam Workshop, or if they did, those people don't work there anymore.
I would've loved Workshop support for Sonic Mania, but the reality of what games look like under the hood and how they get put together isn't all sunshine and roses. I'm sure they would've done it if they could have, but there's no shame that their development tools weren't in a state that they could just put them out there like that. That's true of most developers.