First, the Perl 6 development had bad project management. According to the reports, the culture attracted experimentalists with interesting ideas, but didn't get along with boring people who wanted to do things like releases and steadily approaching functionality. So there were a number of projects, a number of which were abandoned. Things dragged on, and nobody was cracking the whip. It took 15 years for Perl 6 to finally announce its first official release.
Perl 6 also turned out to be too different from Perl 5. Different syntax, and no XS, which meant every Perl module interfacing with a library wouldn't work even if some sort of automatic conversion, or compatibility mode was possible. XS is tied to Perl 5's internals, and Perl 6 was an effort to make a new language from scratch, without using the old interpreter.
Meanwhile, people started abandoning Perl 5 reasoning that since Perl 6 was in development, 5 would eventually die, and the lack of compatibility would mean current efforts on 5 would be wasted.
It took a very long time but finally it was decided that Perl 6 wasn't really Perl 6, but some other, Perl-like language, and renamed to "Raku" to signal this. But by then it was already too late, most everyone had already moved on, and by the time when Perl 6 was finally released it was an extremely niche thing very few people had any interest in.
Meanwhile, Perl 5 development continued in the background, and they decided that it'd jump version from 5 to 7 to avoid confusion. Perl 7 is the direct descendant of Perl 5, with the same syntax, new features and very few deprecated things.
