PoS is more secure than PoW because PoW is susceptible to external attacks by entities who do not have a stake in the network.

If someone wants to attack a PoS network, they need to accumulate over 50% of tokens; realistically, they need to buy those tokens on the market; as they approach the 50% mark, the price goes up exponentially and so does their net worth in tokens (law of supply and demand)... If the stake is well decentralized initially, it is practically impossible for anyone (or any new group of newcomers) acquire over 50% because most existing token holders will keep HODLing the majority of tokens when they see consistent price increases. By the time the attacker manages to accumulate even 10% of all tokens, the price will be so high and they will be so invested in the token that they will have no incentive to continue with their attack. Note that it costs community nothing to defend the network against such attacks; in fact, they profit from them.

On the other hand, with PoW, an attacker only needs >50% of the hash power to break everything. An external entity who has 0 stake in the network could buy up, rent or manufacture ASIC miners to break the PoW blockchain. ASIC miners are not a scarce resource so their price does not grow exponentially when an attacker starts buying them up. If existing miners want to protect the network against such attacks, they are forced to buy more ASIC miners. Unlike with PoS which profits from attacks against the network, it costs the community money to protect the PoW network from attacks. Also, there are no built-in incentives to make an attacker want to stop their attack once they've started. The attacker can profit from collecting mining rewards until they reach the required >50% hash power; they can even sell all the mined tokens as they go so that they are never exposed to the losses which would arise once they complete their attack.