It could be a simple matter of buying larger quantities of .30-06 and .270 Win, due to the popularity of those rounds to typical big box customers. Multi-cases of the popular cartridges, compared with a few boxes of 7mm-08?
Newer cartridges have considerable design costs and perhaps patent royalties that need to be covered by sales, so the factories must recover their costs. Sometimes, new cartridges need special powders that also cost more to produce. The consumer must eat those costs somehow.
The other non-magnum cartridge that is hard to find and perhaps still more expensive than the old standbys, is the .260 Rem. It's even harder to find than 7mm-08 around here.