I think a lot of the "fear of novelty" in autistic people is a rational response to sensory hypersensitivity in a world that isn't accepting of their needs.
For example, I'm a lot more willing to try new foods in situations where I have the freedom to do so in a way that poses lower risk of sensory discomfort or feeling deprived. If I'm in a restaurant and know that I'm only allowed to order one meal, and if I don't like it, I have to eat it anyway or face a nasty argument with my Mom, I'm only going to order someone I'm certain I will like - which tends to be the same thing every single time. Even if the consequences are simply "watch everyone else eat while I go hungry until we pick up lower-quality food for me afterwards", that's unpleasant enough that I'm not willing to take a chance on a new food.
But if I'm at a store with enough money to buy multiple snacks, I might pick up something new alongside a snack I know I like. If I try it and don't like it, I can eat the other snack, no big deal.
The thing is, most NTs could order anything on a decent restaurant menu and expect to receive food that is at least tolerable for them. The same is definitely not true for most autistic people. And unlike vegans and people with allergies, we generally can't describe our food requirements concisely and NT-understandably enough to get good information from wait staff while ordering.
The same applies to lots of other sensory issues besides food. For example, restaurants also carry the risk of being trapped in an overloading environment where you could get in trouble for leaving unexpectedly (because you haven't paid for your food). So maybe you only go to the restaurant you've gone to before and know plays soft music, isn't very busy and has comfortable booths, instead of taking a chance on a new place. Same with going to any new venue, especially if there are any negative consequences for leaving prematurely, or if you might not be physically capable of leaving (eg you can't get off partway through a plane trip, for example).