nikorr, correct me if i'm wrong but i think the only issue is going over 1.5 not under it. the other thing is, a lot of people buy these lower voltage kits to manually turn the voltage up to 1.5 for the sake of pushing the ram's timings/mhz further.
I do understand your point, but from the selling point they have 12 models line up @ 1.35V,
but the numbers of sold RAM sticks, 1.5V is the king.
1.35V is maybe viewed as for "few" enthusiast.
The warranty is voided on the Intel i7 and i5 line if anything else is used than 1.5V and it may have resulted in lower need for that voltage.
Especially when they still have them on sale.
Of course I could be wrong, but if there is less demand, something will be axed.