Basically it's because about 99.9% of the people on earth don't even know that mechanical keyboards exist, and are only aware of the singularly produced "full size" rubber dome boards that come with the computer.
Laptops all come with some form of "near" 75% board, but are often crowded due to space limitations.
Mechanical 75% boards are also crowded so that they can share the 60% case size (in most cases) for production reasons.
In a perfect world, a standardized layout & slighly wider width for a 75% to 80% board would soon become the norm, with "full size" boards morphing into a combo of TKL w/num pad.
What I don't understand is how the 60% boards became the norm for gaming as they have far to many keys for "Just" gaming, yet can't be used as a conventional keyboard .... Try typing a report, resume. or spreadsheet on a 60% board.
Though I personally don't care about appearance, I can understand how the 60% boards have become the predominant alternative size, given the ultra narrow market that they supply.
In any event, I can do anything with my old Race 75%, I have learned to touch type and program on it even though I would love to change things about it's layout if I could.
I'm just glad for the ones that do exist, but wish each model would come out for a reasonable period of time before they totaly disappear. --- It's frustrating to learn of a new/improved 75% +- board only to discover they are sold out, and never come back again "unless in a cheapend, more blinged out version".