I have a few guesses
- Sometimes, complex cards are genuinely worse because they only happen absent a great idea. Like, if you have a unique idea, you make it into a simple card and are done with it. If you don't have a unique idea, you just start tagging on stuff onto a card (and then maybe scrap/re-add/modify) until you sort of have something. This is not always how complex cards are created, but if it is, it may well be justified that they don't do well
- Complex cards have more components and hence more to criticize. Especially relevant if, to whatever degree, you adopt a measure based on how many flaws you can detect rather than the more subjective sense of how excited you are by the card
- When you design a card, understanding it feels like a trivial cost/buy-in compared to the value that the card provides as an idea or if it were/is played with. But if you judge (or even just read other cards) then you have no prior investment into most designs, so trying to get the hang of it can feel like a significant cost. This can create a negative vibe and since everyone secretly decides everything on vibes, that lowers chances significantly
- As a judge, you don't want it to look like anyone can win the contest by force. E.g., say we make a contest and everyone submits a simple design except one person who makes a 5-step traveler line. If you give the win to that person, it sort of looks like you're rewarding the most work rather than the best idea. (This is probably the weakest point though, and in fact high effort cards don't tend to get a lot of upvotes from other people, either.)
It altogether seems like most people primarily like the "oh that's really clean and clever" type cards.