Lack of time is a strong driver for some gamers opting towards simpler games, or at least games whose complexity does not translate into requiring additional time for mastery/understanding. In my circle of gaming friends, as our available time is under greater pressure we've purposely moved away from games that require significant investment of time to those that perhaps retain some complexity but reward a casual time investment.
Another factor I've seen that affects is the demand of contiguous blocks of time to game properly. Some games do not permit breaking up game sessions arbitrarily, so while one might have ample leisure time, there might be multiple interruptions in between which make playing certain games impossible or a burden.