They are a *lot* more expensive. The RPi doesn't come with much out of the box. You need a PSU (many USB ports don't give out enough juice), an SD card, and probably a screen and keyboard to get going. This device is much more arduino like. They're not really comparable at all.
Yes, I'm aware of that. Actually these are being pushed 'free' in limited numbers. But, actually, in pre-history the BBC Acorn computer (which was the beginning of ARM) was expensive too. I feel that qui peut le plus peut le moins who can do the most can do the least is a good way to do it, too.
And a magnitude more of a pain in the ass to set up. To use this Pi, the pupil would require - a monitor or TV, an HDMI cable, a USB mouse & keyboard, a network connection or wifi dongle, a PSU, a charger, an SD card, and a very patient teacher and set of parents capable of setting this all up and transferring files for grading and exercises.
Actually I worked last year with one school that was successfully doing this. You don't really want switched-on smart phones in class, anyway, so you're obliged to hook the micro:bit to a 'computer' (of some kind), as with the Arduino if you want to do any programming.
Also to continue being ranty, Microsoft has had a good look in this time, not surprising since the last two technology heads were biased in that direction. If they want to 'develop', everything, yes, everything (the thing itself and associated ecosystem) needs to be open source, because it's public cash.
I did freelance work for the BBC in the early 2000s, but have stopped (my own choice) now.
Your program is sick! Shoot it and put it out of its memory.