Actually, there are a few reasons why homeless people go vegetarian.
1) While they could eat fast food, even after a few weeks it tends to make you sick. So they go out of their way to avoid meats and grease. Wave a burger under their nose and many have to choke back vomit.
2) Vegan food lasts a lot longer, for the most part, and when it spoils it doesn’t go off with bad bacteria. A canister of generic rolled oats has about 30 - 2 cups prepared meals in it.
3) Of course homeless people tend to smoke, do drugs, and drink alcohol, and eating vegetarian helps to mitigate a lot of the problems of an unhealthy lifestyle. Many also eat that way to “clean up” after quitting the bad stuff.