There may be a nutritional reason, but my very personal and subjective reason is that peanuts don't taste that good compared to the other nuts.

And I don't think I am the only one. Generally, when I put a bowl of cashews and a bowl of peanuts on the table as a snack for guests, the bowl of cashews empties first, most people only eat the peanuts when there are no more cashews.

Peanuts are not bad, and they are cheap, but I'd rather have the other nuts.

I ultramtb. For me it's because the fragments cause me to cough. My rule is no Snickers unless stopped and water afterwards to flush.

It might be preference, when you're running for that long food aversions are common and individual.

A relative runs trail ultras, but I'm just guessing here based on our conversations. About food in general, you learn what agrees with you and what doesn't, and this information is also shared informally. Something might be gassy or tie your stomach in knots. Also, interestingly, the ultra-runners tend to favor "real food" rather than packaged jells and liquids.

My guess is that races don't offer peanuts because of allergies and you want to train using the same stuff as you will get in a race.

This is fairly thin though because you could just carry the nuts in a race.

Generally, ultra food preferences are really personal. The broad facts are: (1) you need a certain level of calories to keep going and it's more than you'd think (though see below) (2) running long distances is hard on your digestive system. The consequence is that it's a challenge to consume enough calories without causing enough gastric disruption that you then can't consume enough.

Within those broad parameters, you see many different strategies, with some ultrarunners preferring "real food", and others preferring engineered food (gels, sports drinks, etc.) Famously, Courtney Dauwalter is a fan of actual candy. Aid stations at a typical race will often have both of these, and at a longer race you'll often see cooked hot food like quesadillas, bacon, pancakes, etc., especially during the overnight sections where it can get quite cold. In general, people try to experiment and find what works for them, which can change as the race goes on.

Contra the OP, there's not much evidence that you need fat intake in an ultra for energetic reasons. Even someone with quite low body fat is carrying plenty of calories in fat to do an ultra. However, some people find that taking in fat helps keep their stomach in line, especially after hours of consuming mostly simple carbohydrates.

There's a huge amount of food engineering work on trying to design nutrition that doesn't cause gastric distress. For example, Maurten pioneered a sports drink and gels that use hydrogel to encapsulate the carbohydrate, which is supposed to reduce distress. In the past few years there has been a lot of interest in the elite ranks in "super high carbohydrate" fueling. The conventional wisdom was that most people couldn't process more than about 75g (300 calories) of carbohydrate an hour, but recently runners like David Roche have been pushing this to 90g or 100g with success, and you see products designed to let you do this easily.

One thing that the OP doesn't really covers is weight. In a typical ultra, you're never going that long between aid stations, so you can pick up food there. Even if you don't like the nutrition the race is providing you can often give them a "drop bag" containing your preferred products which will be delivered to an aid station where you can pick it up. However on an adventure run, you may well have to carry all of your nutrition all the way. The author of the OP suggests going to stores, but this doesn't really work in the backcountry. I've done 12+ hr runs in the Sierra [0] and the Grand Canyon where you just had to carry everything the whole way, and suddenly you become really conscious of the water content of your gels, which can easily be 50% water by mass.

[0] https://educatedguesswork.org/posts/northern-yosemite/ https://educatedguesswork.org/posts/tenaya-loop2/

Maybe a slight allergy that makes it unformfortable to eat.

They aren't nuts but legumes.