Why? The maps tend toward awful, and are borderline unusable in some cases.
See the pictures below and my accompanying notes on how this has all gone wrong. (Please excuse my poor photography; I was in a rush.)On the Hall of the Fire Giant King map I've put a red circle around the room numbers that have a helpful black background/white text for legibility. The numbers in the blue circle (and the rest on the map) are printed in a dark color on a dark background, rendering them almost unreadable. I just happened to choose this map to illustrate this problem; there are quite a few maps in the book that have dark numbers printed against a dark texture. Full-color may make a book look expensive and fancy, but it's murder actually using this for its intended function as a game aid.
The map of the first floor of the Sunless Citadel has over forty locations on it, and the map takes up less than a quarter of a page. That's far too many locations crammed onto a tiny map; I can tell you from actual play experience that it's a struggle to make out the numbers on a map that small, especially when you need to keep flipping back to that map because the description of its rooms take several pages. It was so bad that I ended-up googling a bigger version of the map from an older edition of the game, printing it out, and using that instead.
Surely a full page map will fare better? Sadly, no. The map of the Doomvault has numbers in circles, which seems like a winning combination, but that combination as used here is actually a bad one: the numbers are so small that they're hard to read and, worse yet, the red of the numbers overlaps the red of the circles they're within...so good luck making those out.
Maybe a two-page spread would work better? Well, take a look at the Hidden Shrine of Tamoachan map. Because it's bound in a hardcover, and the adventure is placed near the front of the book, the crease between pages swallows so much of the map that you can just about see that it says SEC(ond) TI(er).
Those maps are straight-up embarrassing. Even a smaller publishing imprint or a DIY effort should have put more effort into the legibility and usability of those maps, so to have the biggest name in the industry foisting maps of this limited utility in their flagship D&D line of products is just sad.
Look, I don't want to be that guy, but I think that when you plonk down your money for a product you should expect better. People cooking up maps for their games do better than this every single day; this is already a solved problem: black & white maps, at a decent size, with contrast between the numbering of items on the map and the map itself.