The real issue is not just that Perl6 wasn’t backwards compatible, it was that Perl6 basically did not exist for real for many, many years. People got tired of waiting, and the lack of backwards compatibility did not help.
Also Perl6 was just more weird on top of weird from a mainstream perspective. Making it even harder to justify.
Nowadays when everyone and their dog (vcpkg) have a package system, it’s easy to overlook how magical CPAN was. A solution to the weirdest problem, just a package away.
It was a magical time.
If they hadn't done Perl 6 the way they did, it'd still be around. Perl 5 was fine but the impending doom gradually let it be overcome.
Even as a die-hard mod_perl fan, I never even tried Perl 6. The language had gone from the most concise and efficient way to process text streams to an incredibly complex OOP language that for some reason had an experimental Haskell implementation, its own custom VM and multiple compilers.