The technical issue is that Linux has no stable internal API for this. Basically, by the time we have ported drivers from one version of Linux, a new version is out and they will have changed a lot of things, and the compatibility wrapper will need a not of changes. And the API surface is rather large (drivers can use whatever they need from all the little things hrovided in the kernel).
It may be easier to port the drivers one by one to Haiku, at least in the long term.