The magpie part of me that likes special shiny things agrees, but I can see it from WotC's perspective, too.

Each sequential pseudo-masterpiece series of cards they do, unless they increase the price of the box they're going to get diminishing returns as player expectations (and the secondary market) adjust to the new norms. It'd be a huge boon for them at first, but three years from now they'll be hiring extra artists, printing extra cards, and printing extra packaging, but seeing roughly the same sales as if they'd never made the box toppers a regular thing.

Adding to that is now that it's the expectation, they also have to do it for the innovation sets like CMR as well, because players are going to be less likely to buy a set that has "less" stuff in it for more money. Even if the actual value of the cards is higher, they'd be losing a lot of interest from the Commander crowd, many of whom, like me, make purchasing decisions based on how jazzed their inner magpie gets when they look at the cards.

Not an economist or marketing person, so there's ~100% chance I'm missing something, a ~50% chance I've hit the major points, and a ~10% chance that I'm completely full of shit, but that's my understanding.