I get why things like hot dogs or bratwurst are readily available as streetfood, it's logistically easy - but so is soup! You need like a pot, maybe two if you're getting crazy with it, maybe some bread rolls and that's it. It's cheap to make, cheap to buy, you could get hot soup on a cold day to warm you up or something like a gazpach or okroshka on a cold day to have a chilling meal. They're stupidly easy to make, all the ingredients basically cost zilch, very easy to adjust for all kinds of different dietary needs if you offer some sort of toppings optionally instead of throwing it all in there.
So why isn't there more soup? It's a style of meal you can find in basically any cuisine yet in all my travels I remember like two instances where I could just get a soup. What drives streetfood and why is soup shafted?
In my experience, most soup places kinda suck. It's not even a dunk on soup, just companies that specialize in it and aren't phó/ramen tend to blow.
Chili tends to make a splash, but likely cause it's better year-round and (being a thicker product) doesn't slosh and splash as much, making it more portable. To add one more factor to consider, is the spoons and cups, making it more of a hassle for both stocking in a food cart as well as easy consumption while on one's feet.