What, other than being a meal replacement shake, is the appeal of Soylent?

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.

To me the interesting part of Soylent is the development and the engagement with the community. Having a blog talking about how it was made and the struggles they had as well as having a place for DIY solutions on the same website as the product they're selling, that's interesting. Food replacement comes second.