Why tokio tasks are run-time dependent ?

Tokio and async-std 

I was going through two major crates for async programming in Rust. I believe Tokio and async-std are predominant crates now available for async programming. 

But when I tried to run tokio tasks with async-std run-time, but default it won't allow. I need to change run-time as tokio using #[tokio::main] that's not nice.

But when  I tried to run async-std tasks with tokio run-time, I am able to run.  Looks like async-std is more compatible with other Rust eco-systems.

tokio runtime with async-std tasks

async-std running tokio tasks error

error:

thread 'main' panicked at 'must be called from the context of Tokio runtime configured with either `basic_scheduler` or `threaded_scheduler

There would be libraries/crates to make it happen, but I think by default this should be available.