Dear Jack,
I agree with you, there are a lot of applications, where the
Z80/180 could replace 8051. But some people design project
with 8051, afterwords may just review(or not) Z80 family data
book.
I know a lot of engineers that could reply "Z80, what is it?",
however, every child learns 8051.
The world is mad about the 8051 with its powerest bit arithmetic.
Even if the microcontroller is irreplacible sometimes, it's not
rational and the best everywhere Z80/180 could be imlemented.
It may be becouse historic reasons. I think the MCS-51 have to say
a lot of thanks to its papa - Intel, and such brothers as ix86.

Best regards,
Oleg