That said, this equation is why I am generally keeping an eye out for extra players. And, as the game has, after two years of play, reached the end of the first semester of the first year of the Strange School (and I have vowed that the game will move faster -- one real year per game year would be great; one real year per game semester is more likely), this is actually a good time for new players to join.
That said, there are reasons that recruiting is tricky. First, as mneme says, I am picky, and this is a good thing. A new player needs to mesh with the group.
Second, the kind of player I like is probably picky as well. This is also a good thing. No one should force themselves to play a game that they don't like.
Third, there's an awful lot of history that accumulated, and this can be intimidating. I've been running stuff in the same gameworld since 1991. The combination of some world changing events and the high school setting mitigate this, to a point. But, I was recently reminded that even the relatively short history of the play by email is longer than I assume.
This is something I was intellectually aware of, but there is nothing like hunting up fairly specific threads that occurred over two and half months of game time and realizing that one is breaking up one's list into three emails so that it won't be too intimidating that drives the point home on a visceral level.