The reason I personally left Perl was the extremely hostile community. When someone asked a simple question, not only would they get an overhaul and namecalling, but so would anyone that tried to help them.
Python seemed to do a much better job at onboarding new people. To me, that seems like the most important reason they won in the long run.
FWIW perl6 takes extra special effort to be nice.
I don't have links handy, but there were official blog posts and such about how they endeavored to fix the problem. Of course I'm not saying it was universal, nor trying to take away from the fact that there are a lot of great people in the Perl community (PerlMonks is a good example)
They did fix it, as far as I can tell anyway, but it was too little too late.