Well perhaps mostly because commercial radio doesn't like to play rock songs anymore, but...
Before we go entirely blaming current pop acts for the guitar solo's demise, I would have to say when you see a lot of the various pop acts like for example, Rita Ora, play live on festival TV coverage they almost always have some shred master general session guitar player come on at some stage and do a bit of wild fret w*nking Van Halen style over one of their tracks. So ironically although it could be perceived that modern pop has killed the guitar solo, its actually modern pop stars that are keeping it alive in their stage shows.
Also has to be said that in the Indie world nobody plays guitar solo's because many of the bands feed off the punk ethic that solo's are self indulgent & get in the way of the song. So that's another reason why you rarely here them outside of hard rock circles now.
I think Muse's Matt Bellamy stands out as one of the few alt rock bands around these days that will play big solo's and yep if people slag off Muse its usually for self indulgence.