Why is hardware multithreading not more common in embedded systems?
In embedded systems, raw processor throughput is not normally as important as determinism; predictable behaviour that can be validated. Multithreaded code is much harder to make deterministic.

It's also only running a single application with a few simple tasks, so multithreading is less of a benefit.

If you need multithreading and more processor power, then some of the medium sized ARM cortex processors start to be very suitable to this.