The nrf51 is available with 32K RAM, but only in combination with 256K of flash, designated with the AC suffix (AA: 16K/256K, AB:16K/128K, AC:32K/256K). The 32K chip has been shipping on Adafruit's boards for a while, and is now even shipping on some of the generic Chinese dev boards and modules, so I assume it's widely available.
I imagine the main reason for going with the cheaper chip is that micropython was a pretty late entry to the list of supported coding environments, and the hardware was probably already pretty settled before the port started. Also BLE+4K of ram is probably usable in the other official (compiled) languages, and micropython does otherwise work aside from something that's a fairly advanced feature. So it would have been a tough sell increasing the cost of every board when most students and lesson plans will never use BLE and there are other languages available if they do want it/need it.
I imagine the main reason for going with the cheaper chip is that micropython was a pretty late entry to the list of supported coding environments, and the hardware was probably already pretty settled before the port started. Also BLE+4K of ram is probably usable in the other official (compiled) languages, and micropython does otherwise work aside from something that's a fairly advanced feature. So it would have been a tough sell increasing the cost of every board when most students and lesson plans will never use BLE and there are other languages available if they do want it/need it.