Perl, once the most widely used scripting language, has been in decline for many years; the decline now looks terminal (many decades from now, when its die-hard users have died), what happened?
Python is what happened. Why was this? Did Perl have a major fail, did Python acquire pixie dust that could not be replicated, or something else?
Some commentators point to the failure to produce a timely release of Perl 6; a major reworking of the language announced in 2000 with a stumbling release made available around 2015.
I think the real issue is a failure for Perl to take off outside its core use as a systems language. Perl is famous for its one-liners, but not for writing large programs (yes, it can be done, but would many developers would really want to?); a glance of the categories in its module library shows; those 174,970 modules (at the time of writing) are not widely spread over application domains (i.e., not catering to a wide audience).
Perl 5 was failing to grow outside its base before Perl 6 began its protracted failure to launch.
Language use is a winner take-all game, developers create more packages, support tools, and new users who combine to attract more developers. Continuing support for minority languages comes from die-hard users, existing software that is worth somebody paying to maintain and niche advantages.
These days, language success is founded on the associated package ecosystem (Go and Rust have minuscule package ecosystems, which is why they are living on borrowed time, other languages will eventually take away their sheen of trendiness). Developers use languages to build stuff, the days of writing the code for almost everything are long gone; interesting software is created by taking advantage of packages written by others. Python was in the right place, at the right time to acquire a wide variety of commercial grade packages.
It’s difficult to see Python being displaced as the lingua franca of software development. Its language features are almost irrelevant, its package ecosystem is everything. The winner will eventually take all.
I’m sure the cycle of languages becoming popular for a few years, before disappearing, will continue. There have always been, and will always be, fashionable languages.