I think there's an element of getting exponentially better as you increase levels. Like, a 3 on the Richter scale for earthquakes might be comparable to a semi truck driving by, but a 4 can shatter your windows.
As a player, If I take 2 turns to get out 2 level 1 cards, when I sacrifice them, I lose their abilities, so I want my level 2 card to have an ability that is worth the trade-off. I don't think this has to be a set-in-stone design principle, but without it, there isn't really a reason to not just have decks that are >90% levels 0-1 beyond other people's deck gimmicks.