Not largely because of Gibson’s dominance. The problem was that about fifteen or twenty pitchers all had ERAs at or below 2.
Actually there were about 10 pitchers with an ERA below 2.2, so “at or around 2”.
Gibson was “at or around” 1.0.
Big difference.
Yes, 1968 was the Year of the Pitcher, but unfairly or not(and there is MUCH that is unfair about the way baseball history is retold...ahem...ridiculous and unfair east coast team and player emphasis..), the bellwhether of that group was clearly Bob Gibson, and many sources cite his off the charts dominance as triggering the rules change.
Here is one:
http://www.baseball-statistics.com/HOF/Gibson-Bob.html