When did that happen?
What was the reason for it?
My theory:
"Maybe this is part of ARM's masterplan to move people 'up' from 'lesser' processors to ARMs...? There would be some sense in it: If your project really needs an OS, then it probably shouldn't be done on an 8051 - and an ARM would probably be a better choice... Especially with the cost of Cortex-M3 devices these days..."
See: www.8052.com/.../read.phtml