A variety of reasons. Some move around as different days brings different crowds. Some only open when they need money and take a break when they do not. Some go out of business as they were just not good at managing their operations yet their food was amazing. Some get sick. Some just move around the block as they were on someone’s turf. Sure there are other reasons but getting shutdown by the police seems to not really happen here much unless you are in front of a government building or a some wealthy property. Out side of that it is a free for all. At least here in Vietnam.