That's because Proof of Work protocols use a lot of energy, and they cost a lot of money to mine because people must buy the hardware. For the average joe, this system cost too much money to get into. With PoS, people just need to buy the coins, age them and then they earn interest. There's also a lot of other protocols out there as well. I just can't think of them. I know there's: PoO - Proof of ownership. I've seen some coin out there do something like this where they would validate a piece of property that you own and you'll have a digital token for that item. I think I heard of another protocol out there that did something with the planets and stars or something crazy or maybe I'm crazy.