Firstly that's not completely true: proof-of-stake doesn't solve the initial wealth distribution problem so often proof-of-work is still necessary in some capacity even in proof-of-stake coins.

Furthermore it's possible that a purely proof-of-stake currency would have never gained the widespread trust that proof-of-work did at a time when cryptocurrencies were a new idea. Or maybe not... but people did have ideas about proof-of-stake quite early on and it took much longer for those ideas to gain the same level of acceptance that proof-of-work did.