What is with the popularity of the 8051 Core?
From Wikipedia:


  Designers use 8051 silicon IP cores, because of the smaller size, and lower power, compared to 32 bit processors like ARM M series, MIPS and BA22. Modern 8051 cores are faster than earlier packaged versions. Design improvements have increased 8051 performance while retaining compatibility with the original MCS 51 instruction set. [ ... ]  Enhanced 8051 silicon IP cores now run at one clock cycle per machine cycle, and have clock frequencies of up to 450 MHz. That means an 8051-compatible processor can now execute 450 million instructions per second.


Some engineers may have learned about the 8051 in school, and so they reach for the familiar part in their designs. (This is like the question: why are new schematics still drawn with the 741 op-amp?)

Also, this is a very helpful, entertaining read which touches on the subject of why isn't something more complex and powerful always chosen in favor of the simple: http://www.danielsen.com/jokes/objecttoaster.txt