I dont think the MCC has much to do with it. The change in batting attitude has had much to do with recent developments in bat technolgy, and One Day cricket that has had players exploit fielding restrictions.
Also dont discout the improved protective equipment. Players in the 80s and 90s will not have been brought up using helmets anywhere close to the level they do now in their development. Now players wear the full works when developing their technique and are able to commit to the front foot earlier and attack wide full balls as they dont have to watch the ball as closely and wait to commit as their is little physical danger.
There have been plenty of other progressions over time. This is just the most recent. Just because something like this didnt happen in the 30s (the above reasons would partially explain why) doesnt mean there was not other subtle changes and evolution taking place.
Shot making and the attitude to running between the wickets has changed markedly in the 30 years that I've been playing watching.
Batsmen are loathe to let the bowler settle and some therefore play more shots, earlier in their innings e.g. Langer went from a relatively stodgy player to a bit of a dasher.
In the 70s and 80s, by far the most common shots attempted when looking for a single were leg-glances and glides through the slip and gully region. Now, the latter, is a rarity when compared to nudges to the leg and off.
Furthermore, smaller boundaries and better bats (although I think the latter is overstated as the top players have always had great bats) have increased the risk/reward profile of lofted shots.
Fitness has also played a role with more players being willing and able to take quick singles or hard run threes than in previous generations
Also, there is not as great a disparity between conditions from one venue to the next. As such, batsmen can be relatively comfortable in the knowledge that a ball will behave in a similar fashion to the last wicket that they played on and hence be more positive.
Finally, by playing so often, batsmen are staying in-form for longer periods whilst bowlers dont get as much time to recover.