It's usually the same principle why black Airpods are cheaper than white ones.
The 750W model is less and less in demand, which means that mass production is less and the price goes up.
In any case, I'd recommend buying the 850W version, the PC will only draw as much power as it needs anyway and you won't have any problems upgrading in the future.
Typical supply/demand principle.
I think that's the reason.
In any case, this product is a good buy!