Jam can be smooth in Britain too, the cheap ones usually are. The opposite, with chunks of fruit, is conserve. In all my years of watching TV, I've never heard an American say the word jam, it must not be very popular compared to jelly.
Given the number of small batch jams available at various farmers markets, my guess would be that for smaller farms, making jam is more practical than jelly.
Probably true? Unless they are cheating by buying bulk commercial (filtered) juice.
But if their customers prefer "I can see lots of fruit chunks in it" jam to jelly, they don't need other reasons.
"Smooth jam" here in the UK is sometimes labelled as jelly, like this kind of thing:
https://www.ocado.com/products/tiptree-blackberry-jelly/1053...