As announced this morning on a live stream, Sega is holding back the PC version of Sonic Mania for further optimization. It's now releasing on August 29, two weeks after the other versions of the game.
Folks who pre-purchased Mania or the collector's edition will get a Steam copy of Sonic 1 for free.
Sega confirmed that the Nintendo Switch, PlayStation 4, and Xbox One versions are still on track for Tuesday. I wasn't planning on going the PC route, thankfully, but I feel for those of you who were (and are now potentially considering buying elsewhere because two weeks will feel like forever).