RTSs attract a certain personality type: the type of person that becomes an engineer, mathematician, scientist, or statistician. I met so many individuals from these career choices when I was playing RTSs that it was uncanny. Given this, it's no surprise to me that RTSs aren't wildly popular, the complexity and depth is just too much, it takes a systematizing nerd (of which there aren't many) to fully appreciate it.
I believe another reason is that these games are mostly 1-on-1, so they don't benefit from the same team play benefits and network effects as other games.
I believe another reason is that these games are mostly 1-on-1, so they don't benefit from the same team play benefits and network effects as other games.