As soon as it became clear that the move from 5 to 6 was going to be discontinuous, if you wanted to be a Perl developer you had to decide if that meant being a Perl 5 developer or a Perl 6 developer. Being a Perl 6 developer was the clearly marked path to The Future, but Perl 6 was years away from being ready to use in production environments, and it never seemed to get any closer. Perl 5 was practical for real-world use, but choosing to be a Perl 5 developer meant worrying that you were investing time and energy learning skills that were already marked as obsolete and writing code that would end up having to be re-written.
That kind of uncertainty is fatal to a language. Why put up with it when there's so many other languages out there with clear upgrade paths? Especially when the upgrade path for the one you're using now appears pretty similar to learning a new language anyway?
So developers drifted away from Perl to other languages, and that, as they say, was that.