With PoW, assuming you can use the same hardware to mine different coins, then you just point your BTC mining rig onto MinorCoin, you get 51% of the hashing power and then you can double spend and let's say crash the value of the coin.

Once that is done, you can move your mining rigs onto the next MinorCoin, or you can put them back to mining BTC without you having lost any value except for the opportunity cost when you weren't mining BTC.

With PoS, you need to buy up 51% of the coins in order to make the attack. As you buy more and more coins, the price will rise, so you will spend more money than you expect to get to 51%. Then, once you're there, you do some double spend attacks and destroy the value of the network. Now, all that money you pumped into it is gone. That is the key difference between PoS and PoW in this scenario.

With PoW, you'd need to pay for enough hardware and electricity to rival half of the hash rate of the entire network. With PoS, you'd need to pay enough to own half of the coins. Both are prohibitively expensive. PoS also discourages you from such an attack as you'd be destroying the value of the coins you own.