Calibres like 35 Whelen and 338/06 have two hurdles.

Firstly there is the 338 Winchester and 375 H&H. The 375 is the start of the big bores and has all the things a 375 just has. The 338 offers about the most power in rifles that are built and priced like 7mm Remingtons and it also offers good velocity.

The above means the 338/06 and 35 Whelen will never be big in the popularity stakes. The only way a calibre can be a commercial success if it is a small seller is if it can be sold at high prices and the 35 Whelen won't bear the big prices. The 30/378 and 378 Wbys etc are obviously very small sellers but they bear very high prices for ammo and cases.

I think when the average shooter steps up above the 30 calibre he likes to see a bigger cartridge than the 30/06.