Ruby's standard library was not littered with too many sync helpers, so making them fiber capable without much standard library effect is a big win. In Python, explicit coloring is required and it's easy to block your asyncio coroutines with sync calls. In .NET, it is nice that tasks can be blocking or not, but there is one fixed global static thread pool for all tasks and so one is tacitly encouraged to do CPU bound work in a task (granted CPU bound fibers are an issue in Ruby too), not to mention issues with changing the default scheduler. In Java, virtual threads take a Ruby-esque approach of letting most code work unchanged, but the Java concurrency standard library is large with slight potential incompatibilities.
Ruby is both 1) lucky it did not have a large standard library of thread primitives to adapt, and 2) smart in that they can recognize when they are in a fiber-scheduler-enabled environment or not.
Granted that lack of primitives sure does hurt if you want to use concurrency utilities like combinators. And at that point, you reach for a third party and you're back in the situation of not being as portable/obvious.