Contrary to popular belief, MMORPGs are a popular genre and they sell well.
Amazon gave up on an expensive and well-funded MMO because it couldn't compete in this area.
Major publishers are less willing to throw around money for an untested MMO when they can wager that effort instead in other proven genres with less overhead.
MMORPGs are more hobby-focused compared to other genres, requiring a time investment but tend to hold attention for considerably longer periods.
MMORPGs make money consistently, however, that does not necessarily mean that there are more people to capture.
MMORPGs are a huge and expensive gamble, requiring a lot of time and money to develop from scratch.
EA has bankrolled two major MMOs post-World of Warcraft that were meant to break into the top MMORPG spot; however, both have failed to do so.
MMORPGs can only sense weakness beyond months or years; the beneficiaries of an MMORPG's declining player base are usually games that are already out.
MMORPGs are more popular than the public thinks, and we should not pretend otherwise. While players like MMORPGs, the genre tends to drag out in big slow expensive games.
MMORPGs have high overhead costs, are a tough industry, and publishers are less willing to throw money at an untested MMORPG, which makes it a huge, expensive gamble.