What do you mean by learnability? It's Linux anyway. What you learn for the Raspberry Pi can be reapplied to any other board out there. The only aspect where compatibility depends on the board is the GPIO part.

Cheapness? Most ARM boards are in the range 50 to 60 USD and I don't know in what world that is not "cheap". Plus, at such prices you can get stuff you just can't buy with a Raspberry Pi anyway (specs wise), so it's not like they are a little more expensive for no reason.

Community support, Odroid is good enough there, while I would not recommend things like the BananaPi.

https://www.hardkernel.com/shop/odroid-xu4/

For absolute beginners, a common platform is essential to be able to follow instructions, in special if they don't have external support and must learn on their own; think kids in a rural town in a developing country.

Trying to follow tutorials for the Raspberry Pi on a different platform will more often than not be a nightmare and demotivate anyone first approaching the device. There's a network effect at play, making the most popular platform also the most desirable for diving in.

As for price, 60USD is almost double the price than a Raspberry Pi 3 (and 10 times a Pi Zero), so it would be double the effort for someone with limited economical resources in those countries.