It is simple you get Market share you get wide development support and a wide range of games, no different from the MD or PS. Car games have been around for years, I think the likes of Horror and music games owe a lot to PC development myself. I remember the amazing Virtual Guitar on the PC in 1992 or 93, you had Alone in the Dark series and you even had the likes of Dr Huazer on the 3DO, The PS just made everything more mainstream and I'm not knocking it for that.
Good point to bring up AiTD, Dr.Hauzer, Virtual Guitar (which I haven't heard of until now tbh). Those games certainly did exist pre-PS1 and I guess with a more fully-backed Saturn more of those type of games would've came to it. I mean Saturn did get AiTD ports anyway, they were even going to get RE2 until Capcom cancelled it internally.
Also, you talk of supporting the MD late, but it got SEGA nowhere. The likes of Comic Zone, VectorMan 2, Ristar Ooze all flopped or sold in low numbers. It was clear that after 7 years of being on the market Mega Drive owners had enough and were looking for new stuff, even the later Sonic games like S&K, Sonic 3 sold in nothing like the numbers of Sonic 1 or 2 despite a massively expanded userbase. SEGA thought people wouldn't have the money to jump on the next gen, but they did and we were ready to jump and that was SEGA's huge miscalculation in the west IMO
There're are few factors here we wouldn't know about; for starters, the actual number of cartridges SoA and SoE decided to manufacture for those games, which would have absolutely impacted total sales. We know SoA limited the production of Phantasy Star IV in America for example, due to it being what they deemed a niche genre, but for all they knew the game could've sold more copies if they manufactured more carts (and playing the game ATM, I can definitely say it would've gained more sales based on the quality alone).
Sonic is more of a curiosity here because I think it was well-known at the time the game was cut into two halves, which may've impacted the number of people wanting to buy it, if they were expecting a full game. Maybe some of them were waiting for a cartridge release combining the two together, but it never came, therefore they never bought either version individually. We simply don't really know on that one, but I'd like to think it's at least part of why those numbers were lower. There's also the possibility that Sonic 3/S&K didn't seem as stand-out to the market by 1994 compared to something like Donkey Kong Country (even if the gameplay were just as good if not better), due to it being "normal" 2D.