I read a really good post a couple of years back about SEGA and why they're such a shadow of their former selves... especially compared to Nintendo.
Nintendo have had, largely, the same leaders in their company for decades. Take Zelda- the original creators of Zelda, and not just Miyamoto, are still there today. Some of them have moved on, a lot of people who worked on the original game have risen in the company. So they are still super passionate about Zelda because it's their baby.
But with SEGA, because the company has changed so much- major cuts after their hardware bombed, a few bad decisions, the company just not being a place that has that "successful future" feel has meant a lot of their original staff, including a lot of the people who worked on the various early Sonic games have moved on. So new Sonic games are being made by entirely new teams, many of whom might have played Sonic growing up, indeed, and therefor have fond memories about it but the passion just isn't there because it's not their baby.
And because the Nintendo high ups are the creators of the original games, their focus is making THEIR game good. The high ups in Sega are more businessmen at this point, somewhat detached and only interested in putting out games and making money.
That's actually part of the reason why Sonic goes through so many changes, too. Mario has remained largely the same character because Miyamoto is like "no this design is good it's always been good", while Sonic gets a major redesign almost every couple of games at this point- because they're just not attached to him. They want to revamp him to hit a new market every time, whereas Nintendo has never lost their fanbase.
