They're low-grade pewter, so cost is not likely the reason.
Balance is possible, but the difference in weight so so negligible that I doubt it. Plus, if anything, the later razors with a solid wedge were lighter than the earlier models with holes & cavities.
The reason that I'm pondering is that the extra holes were originally simply a guide to help with positioning. But I'm mentally shooting down my own idea by noting that these were made from a mold, so there wasn't really any reason to do that either.
So many features of razors are shrouded in mystery!