Fantastic question. Most likely because everyone and their grandma's military has switched to some form of digicam. And it would cost a lot to make the switch, digi works well enough. It's a shame, I really miss the whacky individual Military camo. Every military has some weird form of nanocam now, in ranges of green, beige, brown, and black.
This being said, I still think old school solid color OD is my favorite "camo". Solid colors can be really effective, especially in dense foliage. Since the shadows and patterns that occur on it come from the natural surroundings, you never have to worry about the shadows or movement conflicting with the established pattern on the garment. That, and they natural take on their own natural patterns as you get mud, blood, fading, coffee stains, etc....
Don't remember which channel it is, but he tests out camo patterns by setting the camera up and going and hiding somewhere, then waiting for like 2 minutes and finally moving. It's like grown up Where's Waldo. It may not be fair, since he lives around woods, but I was surprised how easy it was to spot the fancy new digi multicams because of the shadow differences vs just solid color shit like OD.
edit: found him
https://www.youtube.com/watch?v=peBq7snxWFY