[2019] Riot keeps making 1350 skins because people like them.

Nobody wanted a lame 750RP skin (that only changes the model, no VFX or SFX changes) for their main when they've been waiting for a skin forever, they weren't selling so that's why they've substantially been producing a lot less of them.

975RP skins are less popular because the tier doesn't offer SFX changes (that's why Conqueror Karma and Varus was priced as such), or all of the special little touches that go into the work of 1350RP skins (Star Guardian Zoe being a prime example of the quality of this line)

People want a cool skin that changes pretty much everything about their champion, gives them an entirely new fantasy and aesthetic. That's why every skin lately is 1350RP. Because they sell.

And also fun fact, you're spending the same amount of money if you want a 975RP vs. a 1350RP skin. Both of the tiers require a 10$ RP purchase minimum unless you have RP saved over from a previous purchase.