Plenty of technical reasons but for me, the biggest is unity among libraries/technologies. There been versions of coroutines/fibers around PHP for many years, first time I heard about it was a PHP fork that implemented fibers, don't recall the name but must have been something like 10 years ago...
But the problem has always been that you cannot rely on the community having things like that available.
Having it in the core language, means that more libraries can depend on it, and users are therefore incentives to learn more about it, as they can rely on it sticking around.
Overall, I'm happy it's coming into the core language.
Probably thinking of HHVM/Hack: https://docs.hhvm.com/hack/asynchronous-operations/introduct...
Symfony 6.1 and the upcoming Laravel 10 require PHP 8.1+, but I don't see them using fibers yet. We'll have to wait and see.
Performance without downsides.