I don't think it's impossible to include the cream mods in the cream base, but it would increase the price of all kits unless the alternative accents were removed from both Base O and C. As is, the cream mods in Base O are pulled from the cream batch that make up Base C, while the orange mods in Base C are pulled from the orange batch that make up Base O. If we were to make Base C all cream, then Base O would have to be all orange, unless they re-ran some production batches just to make the alt mods and include them in both batches. But re-running some batches means more production, which would increase prices.
The only other way to reduce the production batches and create all the keys that you want would be to re-do the molds to double-up on mod keys, thus creating the extra mod keys that you want to place in the other set. But re-doing molds would also increase costs. This seems to be a limitation of the molds being used to create KBS sets.
So basically, it's all orange and all cream or the current mix that we see. That's how I interpret the situation. Someone can correct me if I'm wrong.