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?
Salmon soup (lohikeitto) is common to find in most Finnish markets and restaurants. Usually exactly as you described, large vats of soup and bowls that are quickly served to each person.
Another perfect food on a cold Finnish winter day is rice pudding (riisipuuro). It has all the advantages of soup but it’s also sticky, so no concern of clumsily spilling soup while wearing thick gloves/mittens.
Ramen, pho, and other soup styles are also common streetfoods (not in Finland) for the benefits you mention