Answer:
The Real Reason: Plain Natives were always on the move, sort of like nomads
What WASN'T the reason: Plains Natives had settlements and never migrated
Step-by-step explanation:
Plains natives were difficult to defeat because they were always on the move, so the U.S. or Mexican Army couldn't catch up to them. While natives who weren't on the move and had established settlements, were situated in a single place which would allow europeans to raid and destroy their settlement.
so, the wrong reason for why the plains nations were so difficult to defeat would be to say they were a situated settlement and were never migratory or "on the move"