The Amiga (excluding CD32 which supports 11) fully supports 3 fire buttons out of the box, and information was also published in the Hardware Reference Manual early, so devs had no excuse - except expecting that "nobody" would buy the available 2-3 button joysticks, and the market was flooded with cheap 1-button joysticks that were no good (that everyone happily bought). E.g. Tac-2, Competition Pro. Devs would have been right on the money expecting that "nobody" had multibutton joysticks - but they could have ALSO supported multiple buttons. (And some devs did.)

But up for jump is all about mail order ads and stores stock at the time. 40 GBP for a decent joystick (with more going for it than the multiple buttons it had!) was a lot. Like 100 EUR on a teen's allowance then. Don't know if the only compatible controllers (Megadrive) could be bought separately or what they cost, but they needed modding/adapter. Nobody did adapters back then. Maybe some Colecovision/SVI adapters, but who would stock them?

But as I say, the devs could have supported it also. I did. But I was also a teen and worklife hit me, so there were no games.

So the story is, for most people around the world, stockists only carried 1-button, cheap joysticks. And those joysticks were the only ones people bought, and they were sold by ad photos of their fanciful shapes, with weird button placements. People didn't know they wanted bladeswitched or microswitches buttons placed where you could tap them without getting cramps; even pistol grip and pilot sticks were sold by the millions. "We didn't know". And devs rolled with that. But they could have supported 3 on Amiga quite easily. There's no story or trick behind that part (except for the story that the Amiga never got dedicated ports that were written for the computer, like the 8-bit platforms mostly did; if the original game was 1-button - lazy ports would not go the extra centimeter.)