Another option is that they were once again mirroring the original Superman, who went from being knocked out by grenades to running the speed of light in about five years. If it wasn't deliberate in the 80s, then I think it's just as much a puzzle as to why they increased in the 40s.
If you start from lower then go up it's far easier than going downward or starting as far out as you can and writing yourself into a corner.
And of course, even the all powerful Superman for the sake of relevance had this moment
I'm not much into that story, but there was something to that plot. The demand for a more reasonable Superman had been in place in some of the fan and pro communities. Long time writer Bates, my favorite really, even worked out a way to nerf him after death, though again I don't think it's as wise to go from high to low.