There are many languages still in use today that have all kinds of warts and ugliness, but they remain in use because they still have momentum and lots of legacy things built in them. So being ugly or old isn’t enough of a factor for people to abandon something in droves.
Once you need to rewrite everything, there’s no reason to stay with something you know since you need to fully retool anyway.
As a Perl programmer since v5 was released, the confusion around 6 completely destroyed almost everyone’s enthusiasm, and immediately caused all new projects to avoid Perl. It seemed like 5 had reached the end of the line, and 6 was nowhere to be found. Nobody wants to gamble so many hours of their lives, and the future of their business, on such an uncertain environment.
If Perl 6 had any visible movement within the first few years, it might have survived, but it was a good decade before they even admitted Perl 6 might take longer than expected, and then more time after that before they admitted it should have been a new language. 6 was interesting for language geeks, and they probably did some cool things, but you can’t run a large popular project like it’s a small research project. That completely destroyed all momentum in the community. Perl 5 development only resumed far too late, after the writing was already on the wall.
Both Bill Gates and Linus understand backwards compatibility as a sacrosanct principle. Python only just barely survived the jump from 2 to 3. JavaScript can only survive this because there’s no other option in a browser.
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.