Yep. I think it's mostly about mass production. Pyra, not really a phone, but can be used as one is the closest thing we have to something where you can install whatever OS you feel like. And it requires a very dedicated effort to bootstrap a project like this and keep costs within control.
Many projects, like Neo900 (or even Jolla Tablet) have had loads of trouble trying to manufacture something that is affordable.
If there was some stock phone with average specs that you could buy, like a Raspberry Pi, then I'd bet we'd see many more options in the same vein as the n900.
I'm using an LG Cosmos 3 [1] and I'd kill even for access to modify the BrewMP OS and re-flash it. That's all I'm asking for! Even that seems impossible.
[1] https://www.verizonwireless.com/basic-phones/cosmos-3/