Because they can't be everything to everyone. Brick and mortar restaurants choose to sell what will sustain the restaurant. They can't have 400 choices on the menu because of the economics of buying all that food. They would end up throwing away so much that it would eat up all of their profits. However, you may have 40 food trucks each offering something very different for the lunchtime crowd. They can each offer a very limited menu from the sublime to the exotic and since they all vie for the same parking spots, every day there are different choices from the public's point of view. Of course when the weather is inclement and it's pouring rain or a blizzard, hotter than Hades or below zero, people are suddenly less concerned with choice than getting indoors and the fairweather clients go back to those brick and mortar restaurants whose food is now no longer too boring. That is unless they have gone bankrupt. Meanwhile, the restaurants may have hundreds of thousands of dollars invested in their businesses in equipment, staff, leases, improvements and insurance - both first and third party. The truck people have maybe 10-20 thousand in investment and really only do lunch and apart from buying food and 3rd party liability insurance, don't have much overhead at all, because most are owner operated. The restaurants are often open at 7AM and don't close until 11PM, yet that lunchtime crowd could represent the difference between solvency and failure. The former have to sell not only their food, but provide a satisfactory ambience, table and take out services and do it at a price that people can afford. The trucks, on the other hand, sell their food for about the same price, wrap it in paper and spend about 30 seconds interacting with the customer. They actually make more profit on each sale than the restaurant.
So, as I said, people need to decide what they want more. It's dishonest to try to pretend that the market will choose and all will be well. The market is not thinking about consequences when they are making their food choices.