The real problem is that you can't get a phone [1] which can run upstream kernel. So while there's postmarketOS and Replicant (both trying to port regular Linux (for lack of a better word[2]) to phones), very few phones work properly (and most are fairly old and not sold anymore, IIRC Replicant's newest phone they support is the European S3 - guess how old that thing is).
So now, for each device, you have to port desktop Linux [3] to whatever generation kernel it came with.
Debian, in contrast, can assume that you're using the standard modern kernel.
[1]. I mean that it should work as a phone, not just a small tablet. So if it can't make phone calls, it ain't a phone. All the more so if you want a working GPS and Camera.
[2]. I would have said Gnu/Linux but postmarket is based on Alpine which is based on busybox/muslc and not gnu.
[3]. Anything which issues syscalls is fair game, so, for example, if your oldest phone you want to maintain is six years old, then you have to back port every single program (and library!) to work on a random seven year old kernel.
Even back porting android user space (which is mostly Java and doesn't interact directly with the kernel) is a huge pain (and sometimes is actually that hard that the maintainers just give up). Porting back Debian?
postmarketOS tries to bring a desktop-style, Alpine-based Linux distro to mobile devices. It has some level of support for relatively recent (~2015) devices, but there's still a lot of work to be done.
Replicant is working on creating a fully free software distribution of Android, rather than trying to provide a desktop-style Linux.
Porting programs isn't generally a huge deal. Most devices on postmarketOS currently use the old downstream kernel, and most things run fine on it.
The main problem is that a lot of peripherals require closed source drivers and firmware. For instance, very few devices have support for hardware-accelerated graphics or wireless connectivity, both of which are necessary on a mobile device.
So now, for each device, you have to port desktop Linux [3] to whatever generation kernel it came with.
Debian, in contrast, can assume that you're using the standard modern kernel.
[1]. I mean that it should work as a phone, not just a small tablet. So if it can't make phone calls, it ain't a phone. All the more so if you want a working GPS and Camera.
[2]. I would have said Gnu/Linux but postmarket is based on Alpine which is based on busybox/muslc and not gnu.
[3]. Anything which issues syscalls is fair game, so, for example, if your oldest phone you want to maintain is six years old, then you have to back port every single program (and library!) to work on a random seven year old kernel.
Even back porting android user space (which is mostly Java and doesn't interact directly with the kernel) is a huge pain (and sometimes is actually that hard that the maintainers just give up). Porting back Debian?