Good points made already, but the biggest downfall, as mentioned, is the fact that you have to have a lot of coins to start a masternode, and it can often be expensive. With PoW (mining), you can mine from zero coins and earn.
Don't get me wrong, I love staking and masternodes, but you do need that collateral to start a masternode off.
The middle ground? PoS, staking. Stake as many coins as you have and earn.

-Knightly