Personally, I think it's a number of things:
1) People not having the time for IRC anymore
2) people getting tired of the crap that seems to go from one channel to another on the same network
3) people not wanting to move to other networks
4) other forms of communication, be it social network sites like facebook or IMing
5) as has been pointed out to me many times by my job, those who are happy never say anything
6) IRL
and there's tons of other reasons I'm sure. For me, I dont use mirc at all, tho keep updating so IF I go back, I'm all up to date. My real only social medium is Facebook (and possibly google+ if I care to start a profile there)
