With food trucks, first and foremost, people don't "hang out" for as long as they might at a restaurant. It's more akin to fast food - get your food, wolf it down, get out. The process is also different - people approach a window, order their food, and then eat it wherever they want. They have the option of eating it near the truck, but the convenience of taking your food to a different area of your choosing (say, your warm and cozy house) is also there. In addition, in places where food trucks are hugely popular like Portland and Austin, there are "food truck parks" where multiple trucks congregate around a shared communal eating area that is often covered and heated. That space is often operated by an entity other than the food truck so the truck operators don't have to worry about it.
Restaurants, on the other hand, are places where people like to hang out at for extended periods of time. Whereas at a food truck you might order all of your food at once, at a restaurant you're ordering your first drink, then an appetizer or two, then another round of drinks and your entree, then more drinks and your dessert. That process lasts a lot longer than your time at a food truck. There also isn't a convenient "communal area" like at a food truck park that is in the center of a block of restaurants that restaurants can send people to - restaurants must figure out how to build their own seating areas and provide ample warmth.
Food trucks don't care about people being comfortable because that's not in their business model - you order your food and where you eat it is your problem. In order for restaurants to maintain business, they need to cater to the comfort and warmth of their customers.