The TRS-80 and similar system have something no other system has:
* A simple DOS like system (Linux is just a steep learning curve)
* Simple IO communications
* Easy hardware maintenance
* Self-contained system
Raspberry Pi is a step in the right direction but if something goes wrong I can't soldier in a new RAM chip or pull the processor. People really like a simple all-in-one unit. The wide range of choices and lack of uniformity makes it hard design a solid education program.
10 to the minus 6th power Movie = 1 Microfilm