I would presume it's because they're low in sugar. Due to exploding diabetes rates, Mexico has been making a concerted effort in the last few years to stem the consumption of sugary foods, drinks and snacks, particularly amongst kids. You can't have a cartoon mascot on a box of cereal, for example. They put big stickers over Tony the Tiger before changing the packaging completely. And the cost of snack foods has skyrocketed, making it largely unaffordable for lots of Mexican families. A bag of chips there costs more than it does in North America.
My guess is that this is part of that effort.