Why do all meal replacement drinks have sweet flavours?
About 10 years ago, a store I shop at regularly (North American style grocery chain) had some powdered high-protein low-fat meal replacement 'soups'. Their prices had been greatly reduced. They were approaching their best-before date in a few months and the store was clearing them out before they had to be tossed. So I bought them all.

There were two flavours - asparagus and leek, both in a creamy base. They were meant to be eaten the way a soup is but were thin enough that they could've been drank if a person wished.

I asked why they were so cheap and was told that people weren't interested in foods like that. It was the sweet meal replacement drinks already prepared that sold. Since what I bought was a powder and had to be mixed with hot water to properly disperse the powder to make a soup. It seemed the vast majority of people didn't want the 'bother' of mixing the soup. They wanted it ready to drink immediately. And most preferred a sweet drink rather than a savory soup.

Even though I regularly shop at Asian markets (even more than I frequent than large Western type supermarkets), I've never seen anything that's the equivalent of meal replacement drinks. I can't speak for what European stores carry but the concept of meal replacement drinks seems to be foreign to Asian cultures as far as I've seen.

These are two likely reasons (sweet taste and convenience) that savoury meal replacements haven't done well (in North America, at least). One other thing worth mentioning is most Asian cultures (outside of India which has a rich history of sweets) don't eat anywhere near the amount of sugary foods that people in North America do, although it's changing over among young people. Asians consider all the sugar unhealthy.

(I don't have references to back up my last comment but I've had many Asians mention that to me in conversations about food.)