I believe, as part of the Loop Streets projects, the sidewalks along Main were widened, making the road narrower. As said in the article, that was the thinking then - wider sidewalks = better environment for pedestrians = more pedestrians (forgetting that tearing down everything else would have the opposite result)
I think thelakelander hit the nail on the head about State/Union, it has probably been a one-way pair since the Mathews was built in the mid 1950s., and the alternative would be an expressway - a much more "solid" barrier.
Regarding Ocean Street being cut off - there was a proposal back in the 1970s to extend it across the park, but local opposition to carving up more of the park stopped it.
About the JTA station cutting off Downtown/Springfied access - I believe FCCJ was there long before the JTA "FCCJ Station" was built. Although, if the college opens up Hogan Street (which would require a change of attitude, given the tall fences all the way 'round campus), then the JTA station would be in the way.