Vitalik had trouble solving the "fork" issue, that is, that when two forks arise in a PoS coin, there is no incentive for PoS miners to select one over the other.  That is, they are incentivized to mine both simultaneously at the same time.  He thought he may have solved it today, but didn't seem terribly interested in pure PoS, especially because you need to typically do a 100% premine.

There are other people I know who previously wanted pure PoS chains arguing about them for one reason or another now.  I would guess that Satoshi himself/themselves also considered this (as it seems like the more logical means to construct a cryptocurrency network), but abandoned it for whatever reason.