Proof of Work is harder to ban than other consensus like Proof of Stake. In PoS you cannot reliably prove if someone is DDoSed or is only pretending to get advantage by slashing. In PoW if there is a monopolist, it has to keep working to hold that position, in PoS the validator can sign every competing chain and always be rewarded. In PoW when someone has 51% computing power, then turning off some miners may even be profitable, so it is natural pressure against centralization (imagine you have 100% computing power, then there is no reason to mine blocks starting with 80 zero bits and pay high electricity bills, you can save money by letting the difficulty drop). In PoS when someone has 51% coin supply, then there is no reason to give up. Once 51%, always 51%, it is true in PoS, because you can sign every chain using different addresses and pretending that you are not a monopolist and you can still control everything without making any additional effort, like in PoW where you have to keep your mining equipment running.