To control the majority of hash power, you must put real resources and might still fail due to slow in movement, it is much more difficult than controlling a few stake holders. Imagine that once bitcoin is becoming POS, then top 10 stake holders will immediately enter the watch list of CIA
Money can change ownership much quicker than mining rigs. It's much easier to find a mining farm if you want to control Bitcoin (hint - it consumes a lot of electricity).
Bitcoin has a warning system and block IPs to warn of it happening. What does PoS have?