There are way, way too many champions whose kits are relatively fine but whose models and animations are still living in 2012. Olaf's got no fingers, Malphite is still a nose, half the yordles still look like they were smashed with a hammer, _all_ of the hair on base models pre-Yasuo is atrociously modeled, etc.
I think that's a major factor in skin sales. Champions whose base skeletons are better to build from make for better skins and consequently sell better. If they'd do the work to make sure they have a better base model to build up from, then they could give better skins to more champions.
And the issue extends to the animations as well. Not just the _quality,_ though some are incredibly stiff, but the actual fighting style of many of the characters as well.
Take, for example, Master Yi and Yasuo. Both have similar swords and fighting styles. Yasuo's is little less stylized than Yi's, but they still make for a good comparison. Yasuo's stance and attack animations closely resemble an actual katana demonstration. Yi, conversely, stands with a two-handed stance despite only attacking with one hand, and the way he swings his sword doesn't resemble any actual martial art. It looks like the work of someone who didn't use any references.
Another, and in my opinion worse, example would be Xerath compared to Lux. They both have a ton to work with when it comes to their particle effects. But Xerath's base floating animation is extremely static. Barring old Kayle, he has the stiffest animations in the game. Almost as if the entirety of the time spent animating him went into his dance. Lux doesn't move much either, but it's at least enough to make her seem alive. Hell, even Orianna and Blitzcrank are less mechanical than Xerath. Lux gets skins based almost entirely on the fact her particles are so welcoming to them, but Xerath would work just as well if not for how little he actually moves.
It'd definitely take a lot of work, but I think in the long run they'd see a lot more sales even from people who aren't begging for years for their niche main to get a new skin. And the creative team would get a much-needed break from working on the same 30 champions.