"I understand that postpartum many new mom's want their own mom around."
I think that is a fair assumption, and quite natural for a new mum to want, and feel comfortable, having her own mother to help in those early days after having a baby. And it's quite natural for a paternal GM to feel a bit left out in those early days, but most of us do understand, and probably did the same ourselves.
"But if a MIL is also willing to help then isn't the son also worthy of his family's support at this momentous time?"
Of course the son is also worthy of his family's support, it is also a momentous occasion for the DS of the paternal in-laws. It is, however, a different kind of help/support, that is required by the new mum, who probably feels more comfortable with her own mother. There is no reason why the paternal in-laws can't/shouldn't support their own son, and quite understandable that they would want to share in the joy of a new GCs arrival. It is a sad fact, however, as seen in some posts, that some DiLs would prefer to exclude her partners parents. It may all depend on the kind of relationship they already have, or simply on the type of person they are.
"And what about ongoing involvement?"
If the family all get along together then there is no reason why there shouldn't be ongoing involvement with both sets of grandparents. Everyone needs to respect each others feelings, and not overstep the mark, or be deliberately selfish by excluding either set of GPs.
"It seems rare that things ever even out with both grans. Is this a historical bias or a recent one?"
I don't think it's historical, or bias, it's simply down to a number of factors, such as the kind of relationship you already have with your in-laws, distance involved, and whether or not GPs are still working, or retired. Many factors dictate the kind of involvement, and how much involvement, GPs have with their AC and GC.