I was looking over the list on mlb.com and 8 of those have come since I was born (1967). I think this is in part because big league hitters try to hit more home runs now and strike out so much more than in the past. There is less focus on putting the ball in play, and so it is less likely for a poorly hit ball to find its way through to becoming a hit.
The increase in extreme performances, like no-hitters and 60 homerun seasons, is a statistical phenomina related to the overexpansion of Baseball. The talent is spead so thin that the true stars outperform the "average player" by a wider statistical margin that, say, prior to 1968. Look at the spread between the Highest & Lowest individual batting averages, HR's and ERA's for seasons before and after 1968 and you'll see my point.