Why? One reason why is that huge a jump (34-70) will have really challenging shifting. Another, and perhaps the best answer, is that companies only want to make what a lot of people will want to buy (be the motivation from marketing or experience). But if you were able to order enough a quantity (like many thousands I suspect) I bet a company could be convinced to make them for you.
I sense an underlying aspect is at play here but not yet reviled, the OP's cadence preference
Andy.