Marketing is one, but association is another. I think most people associate SEGA with Sonic, and there's some very... diverse feelings on that. I remember in the last five years I've been in NeoGAF, during 2011-2014 especially, there were a lot of people who would come into SEGA threads to shit on them and say they made crappy games, and this was after things like Valkyria Chronicles, Vanquish, Bayonetta, Yakuza series, Binary Domain, their good PC remasters, Sonic Colors/Generations, and others had come out.
I think some people just really dislike them for some reason despite SEGA trying their damnedest last generation to do what no other big company was trying to do; make innovative fun quirky games for gamers. I think they were one of the strongest studios last generation, but last generation was a lot of people talking the shit out of SEGA being a terrible studio, and I don't think just here. Mixture of bad marketing to the general public, and the 'core gamers' their games were more designed for than a general populace and those who follow releases have a negative opinion on SEGA during the time.
So if the marketing is bad to let outside consumers know about these games, and if the more rabid game followers that would post on forums about games have some overflowing loud negative opinion on them, then they'll probably sell badly, since most won't know about the games, and many who know might not give it a chance due to brand recognition in a negative light.