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?
It can be done and is done in Finland, or at least Helsinki -- there are a bunch of stands where you can get a salmon soup by the docks. I think a would-be soup vendor in, say, burgerland, is up against cultural expectations about soup, namely that it comes in cans and sucks and you only eat it when you're sick.