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).
PoS currencies naturally decentralize as more and more users join the economy. This makes it increasingly harder to control a certain percentage of the stake because it requires you to control an ever increasing amount of people.
PoW currencies naturally centralize around large mining groups/corporations as the economies of scale push out smaller miners. This makes it increasingly easier to control a certain percentage of the hashpower because it requires you to control an ever decreasing amount of miners.