Cortex-A vs. Cortex-M, Real Time Applications

I'm searching for microcontrollers for a real-time motor control application. I see a lot of Cortex-M solutions for handling this, but why don't we see a lot of Cortex-A solutions specifically targeted for this?

Is there something inherent to the Cortex-A that makes it inappropriate for hard real-time applications? I'm also aware of the Cortex-R, but there aren't as many vendors providing solutions using this core.

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