Genesis got it's start by competing with the NES. Technologically NES < Genesis < SNES. However this is the time frame the 3 were released back then. Genesis had the jump start, as a result, it had lots of customers already.
Not to mention the Genesis Does adds, and all that.
Not to mention the Genesis has lots of good games as well, and people with Genesis' knew that, and bought those games. Graphics weren't everything, and some may argue to this day there is better games on Genesis than there is on SNES. Depends on what you like really, and that plays a big role. Both consoles were good.
Going beyond this, Sega screwed up, and ticked off, and confused their consumers with things like the Sega CD, and the 32X, and a planned Neptune (Standalone 32X).
Come the time for the Sega Saturn, the console wasn't even that bad (I have one). It's on Par with Playstation in my opinion. The problem was marketing. Back in the Fifth generation of games, we had all sorts of companies trying to come in on the gaming scene (kind of like in the newest generation). You had the 3DO, the Jaguar, the CDi, the list goes on. What happened here was people were going to stick with what they know. Sega, and Nintendo, never mind Sony right now. Anyways, the Saturn planned to get a jump start on Nintendo again, and they set a release date. A release date Sony matched with their Playstation. Sega, seeing this decided on the unveiling of the Saturn, to announce that the Saturn was on shelves right now, 6 months early. This was a last minute decision, and pretty much is the biggest mistake they could have ever made. The people making games for the Saturn were shocked, and not ready, meaning little to no launch titles existed, the stores weren't notified, making it difficult on them, and the people got what was at the time, a sub-par unscheduled product. This ticked everyone off, and they all went to the other guy, which would have been the Playstation. This includes Stores, Gamers, and the game creators. Hence forth is a big reason why the Playstation got a big kickstart.
Dreamcast made an attempt to redeem the company name, and did a farely good job, but too little, too late. A DVD player might have saved the console, but who knows.
There's a history lesson for ya. Sorry haha, I've researched a lot into the matter because A. I lived it, and B. it interests me.