Other products exist which can replace all food. Naso-gastric tube feeding isn't new and people spend long times on that. These products exist in a variety of sweet or savoury flavours (or even "unflavoured"); they exist as shakes, soups, or "juices"; you can get them in little bottles or big tubs of powder; they cover a range of different needs (weight loss; weight gain; different compositions for people unable to cope with certain nutrients etc).

It's nothing to do with developing world. WFP already have a bunch of product which is cheaper, already being produced in bulk in the local regions, and doesn't need as much clean water.