You mean like symbian or s40? Well RTOS are more expensive to maintain and especially to extend than linux (Linux kernel provides all necessary services, for your own custom RTOS you have to do a lot by yourself) Also I would argue linux has better performance (less communication overhead), but honestly i'm not an expert.

Nokia won't give up on s40 for "dumbphones", but maybe they decided they could reach lower price points with linux too and don't have to use s40 for that.