the liability thing is a big part, but the other side is that there are people who will and have abused landowners. case in point...
i hunt a spot next to a couple of fields with a lot of hogs. they're there mostly because i have caused the survivors to move there. naturally i tried to get permission to hunt it by seeing if his neighbors could get me in. a reference from a land owner's friend is the best back stage pass i've found. however, the guy just didn't want any hunting.
then a neighboring landowner asked me to get his kid a hog. i said his buddy's unhunted field was the best chance of a guaranteed kill for a kid with minimal effort. they were best friends and the guy still didn't allow it.
finally i saw the land owner one day and stopped to say hello and introduce myself as the guy everyone was calling him about. turns out he's a super nice guy but explained that he wasn't even letting his family members hunt. he said over time he has had panel boxes, tractor windows, truck radiators and pivot tires shot up. because of that, he doesn't want anyone hunting, period.
poachers have no respect for property, wildlife or laws. they don't ask. they just take. then there are the folks who say all the right things and give a good first impression, get to hunt, then come back without permission and bring their friends. there are even people who have permission who cause an accident but won't own up to it. i suspect most land owners in prime hunting spots have seen it all.
so, while i don't always understand a land owner's decision, i've learned to respect it. after all, they have the right to refuse hunting to anyone and everyone for any reason or no reason... AND the right to file an insurance claim if the crop fails