Every device requires a maintainer, someone who builds Lineage specifically for that device.
On top of that most of these OEMs don’t provide device firmware drivers publicly (camera, modem, speakers, etc), so the maintainer has to either use a generic driver (which typically sucks), or reverse engineer something more suitable.
It’s just a time and effort thing. As with all open source projects it relies on the community to volunteer their time for the benefit of others.