It's quick easy money. Easy to develop; takes a minimum amount of staff away from other games. If mobile gaming really is the future (as in, it will destroy console gaming completely) then it's the smart move. They need to freakin' accommodate Android too, though.
These kinds of games aren't what keeps SE from working on the games that we really all want, it's the big games that fall behind or have to be completely reworked (e.g. FFXIV) that do that.