1. How secure is PoS?
POS is distributing coins by staking. Although it's quite secure than POW but it doesn't have a real consensus in it's true sense.
2. If its so secure why are most cryptos still running on PoW?
PoW is the vanilla form of mining since its derived from bitcoin. PoW has it's deterministic way of assessing supply vs demands.
3. Does this mean that ETH blockchain is not as secure as it is said to be?
ETH and ETC are different. The PoW attack occurred on ETC blockchain. PoW attacks generally occurs when major involvement and popularity is not there in a project.