Some airlines only use one type of aircraft on any particular route, so (since the aircraft are all the same) the seating pattern on any particular flight will be known well in advance.
Other airlines use several different types of aircraft on each route, with different seating plans. Operational reasons (such as having to replace aircraft with faults) mean that those airlines won't know, with any degree of confidence, which particular type of aircraft will operate for a particular flight until a few days beforehand.
So it's easy for some airlines to allocate seating well in advance but impossible for others to do so.