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.
What's an example of a language that's as bad as Perl, that's used as widely as Perl was, that's still in use today?
Perl died because everyone who mattered knew it was a bad language, knew it had to undergo drastic changes, but that essentially meant implementing a new language.
Perl 6 was the symptom, not the cause.
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Note: by "bad language", I mean bad for anything much beyond the kind of thing you might use awk for. It was bad not because of subjective aesthetic issues, but because it was difficult to maintain non-trivial code bases. Its "write only" reputation was well-deserved - the original author of a non-trivial Perl program might be able to maintain it, but once a team is involved, forget it.
PHP? I don't know how widely it's still used, but I'd guess it's more widely used than Perl. Also, PHP is not "as bad" as Perl. It's much, much, much worse. It's Perl without the charm.