You have to prioritize something. Development of other features was prioritized over async first. Futures support came later, and async later still. It's an iterative approach. In the end, it let people get a LOT of value out of rust well before async was fully baked.
For myself, I'd been waiting for the syntax to finalize as I'm still learning and didn't want to really delve into old and new ways, though I'm sure I will see them in practice. For others, depending on your needs, if you didn't need it, or willing to jump through the hoops, you could still get value along the way.
Futures were developed outside Rust core, in a third-party library, before being brought into the language. Working with them in combinator form definitely was less ergonomic, but async/await fixes that.
That version of Rust also did async I/O in the runtime. Async I/O has always been part of Rust. The model changed because there was too much overhead doing it the more ergonomic way and it got booted out of the runtime.
For myself, I'd been waiting for the syntax to finalize as I'm still learning and didn't want to really delve into old and new ways, though I'm sure I will see them in practice. For others, depending on your needs, if you didn't need it, or willing to jump through the hoops, you could still get value along the way.