Is there an in universe reason why pumpkin juice is very popular in the wizarding world but not in the Muggle world?
Out of universe, I understand why JKR would pick pumpkin juice over orange juice: Pumpkins are heavily associated with magical properties; people associate them with Halloween and witches.
But in-universe, is there any reason why pumpkin juice is consumed far more often than by Muggles?
The series is set entirely in Britain, so pumpkin juice is known to be popular with British wizards, but not necessarily in the wider wizard world. It may simply be a popular drink in the British wizarding world because pumpkins are a crop that can easily be grown there, so because it’s easy for them to grow it in the natural conditions of their native country, they find more ways to incorporate it in their diets. Generally, cultures use foods they can grow in their climates in their traditional foods, and pumpkins are something that can be easily grown in Britain by the wizards living there without a lot of effort to get them to grow. We see Hagrid growing his own pumpkins at Hogwarts.
“Come an’ see what I’ve bin growin’,’ said Hagrid, as Harry and Hermione finished the last of their tea.
In the small vegetable patch behind Hagrid’s house were a dozen of the largest pumpkins Harry had ever seen. Each was the size of a large boulder.”
- Harry Potter and the Chamber of Secrets, Chapter 7 (Mudbloods and Murmurs)
Pumpkins are also used in other wizarding world foods like pumpkin tarts and pumpkin pasties. This as well could be because of its availability and ease of growing it. While it’s surely possible for wizards to grow oranges in places like the Hogwarts greenhouses, they aren’t suited to the climate in Britain so would need additional care and adjustments to grow there. Pumpkins would be easier to grow in Britain - Hagrid grows his in a normal patch, though he uses magic to grow them bigger.
Though pumpkin juice is certainly more popular, orange juice is also available in the wizarding world. Harry drinks it in the Great Hall a few times, so it’s something that wizards do have access.
“Thanks, Hedwig,’ he said, stroking her. She hooted sleepily, dipped her beak briefly into his goblet of orange juice, then took off again, clearly desperate for a good long sleep in the Owlery.”
- Harry Potter and the Goblet of Fire, Chapter 15 (Beauxbatons and Durmstrang)
Hermione also has orange juice while eating in the Great Hall.
“Hermione, however, had to move her orange juice aside quickly to make way for a large damp barn owl bearing a sodden Daily Prophet in its beak.”
- Harry Potter and the Order of the Phoenix, Chapter 12 (Professor Umbridge)
So, they don’t lack orange juice - they just prefer pumpkin juice, and it’s more traditional.
There’s no clear in-universe reason why Muggles in Britain don’t drink pumpkin juice. We can only speculate - Muggles should have the capability to make it, but it’s not really clear why they don’t. Perhaps it seems like an odd thing to Muggles but not to wizards.
The reason why wizards typically like pumpkin juice and Muggles typically don’t could be that good, while wizards mainly eat the same sort of food Muggles do, they do have certain foods that while Muggles could make them, most Muggles would never dream of eating on purpose. For example, there’s Bertie Bott’s Every-Flavour Beans, which can be flavored like regular jelly beans, but some taste like odd foods like liver and inedible things like earwax and grass.
“You want to be careful with those,’ Ron warned Harry. ‘When they say every flavour, they mean every flavour – you know, you get all the ordinary ones like chocolate and peppermint and marmalade, but then you can get spinach and liver and tripe. George reckons he had a bogey-flavoured one once.”
- Harry Potter and the Philosopher's Stone, Chapter 6 (The Journey from Platform Nine and Three-Quarters)
While some are flavors Muggles would enjoy as well, and Muggles would certainly be capable of making jelly beans in all the flavors Bertie Bott does, Muggles would typically not want them.
“They had a good time eating the Every-Flavour Beans. Harry got toast, coconut, baked bean, strawberry, curry, grass, coffee, sardine and was even brave enough to nibble the end off a funny grey one Ron wouldn’t touch, which turned out to be pepper.”
- Harry Potter and the Philosopher's Stone, Chapter 6 (The Journey from Platform Nine and Three-Quarters)
In addition, there are wizarding sweets like Acid Pops and Cockroach Clusters, which both could conceivably be replicated by Muggles if they wanted. Both are in the “Unusual Tastes” section at Honeydukes, and it’s unclear who actually eats them on purpose, but they still are sold.
“And the Acid Pops? Fred gave me one of those when I was seven – it burnt a hole right through my tongue. I remember Mum walloping him with her broomstick.’ Ron stared broodingly into the Acid Pop box. ‘Reckon Fred’d take a bit of Cockroach Cluster if I told him they were peanuts?”
- Harry Potter and the Prisoner of Azkaban, Chapter 10 (The Marauder’s Map)
It’s possible that wizards, due perhaps to their slight physiological differences to Muggles (the ability to use magic is one), also enjoy a few tastes that Muggles wouldn’t, which could be why they drink pumpkin juice and eat Bertie Bott’s Every-Flavour Beans and Muggles wouldn’t.
(Thanks to Valorum for inspiring this by saying ‘I don't drink pumpkin juice because it tastes vile!’)
I think as pumpkins are associated with Halloween, and therefore witches, it would definitely be on J. K. Rowling's list. And I also think her own personal preference would have a role. If she liked pumpkin juice, she might (consciously or unconsciously) have the characters have it as a favorite.
