Why did Kennedy do poorly in Ohio?

[Badger]

Ohio was much more Republican at the time in general, this really didn't change until the 1990s

Ohio's unbroken, leading bellwether streak dates back to 1964. Since that presidential election cycle, Ohio has been carried [by all winners] with a percentage margin within five points in spread from the national numbers [in the U.S. Popular Vote].

That actually doesn't appear to be the case. Kennedy underperformed in OH compared to all subsequent Democrats (though the PVI wasn't +5 R; it was under +4). Just 4 years later OH was +2 D, and in subsequent elections it consistently held a PVI that was around +1 PVI give or take a fraction of a percent (Bush in 2000 barely broke +2 R PVI, and OH was actually about half a point +D in 72). The PVI was pretty consistent post-1960.
Ohio went from a Cook PVI of about R+5 in 1960 to only R+1 in 2012, with most of that change coming during the 1980s and 1990s.