Last I used Lineage (a couple years ago), device support was entirely community-driven. Basically someone has to come forward and say "I want to be the champion for device X", and then that person or Lineage had to have one of that device physically on-hand for testing, and then that person and any others who volunteer are responsible for porting each new version to that device and maintaining compatibility.

So the spread of devices tends to be based on how popular/beloved that device is by the community. This can partly have to do with price/specs, but it also has to do with how easy it is to maintain (does it have a locked bootloader? is it available internationally? does it have weird, proprietary drivers that are hard/impossible to get working?).

For example: the Samsung Galaxy S 3-5 are still supported. The S3 came out in 2012 (it was a great phone with an unlocked bootloader; I had one). On the other hand, I don't see any mainline Galaxies newer than the S5 on the device list.

OnePlus tends to be well-supported.

Nexus devices were well-supported, but perhaps the Pixels have gotten more locked-down? I haven't been keeping up with that sort of thing.