Because troops want more variety not less, and because meal replacement drinks have existed for years. The only major difference with Soylent is their target market.

IIRC one of the big things they had to change with the MREs was to add more variety. Troops get low morale eating the same thing over and over.

My grandfather fought on the front lines in the Pacific Theater during the War.

Being at the very end of the supply chain, they got the worse, least desirable MREs after the variety had been cherry-picked by everyone else upstream. I forget exactly which ones they were but it was pretty grim.

Luckily, it led to the creation of the MRE pizza. Apparently pizza was the #1 most desired MRE.

You'd have a mutiny on your hands if you tried to force all the troops to consume soylent. In no other endeavour is morale more important than the military. These people carry automatic rifles all the time.

Not for the troops, for the detainees.

When I did the service one of the benefits working weekends was extra good food. It is good for moral. Soylent on the otherhand...