Mmm to be fair, North America has absolute garbo mobile infrastructure overall, so it might be that... the companies literally can't afford uncapped internet for everyone, because their whole network would come down. Considering how often people are like "yeah, we couldn't reach anyone b/c all the lines were busy" basically never happens here, and in Finland we actually have nationally mandated network redundancy and internet is considered a utility like water or electricity.

But the likelier answer is just North American Gold Rush Capitalism, because IIRC, speedcapped internet is also the norm in pretty much every other continent. Once the infrastructure is built, it is more profitable to have as many people on it at cheap prices to keep the money for upkeep rolling in. It's the difference between making all of the money, right away, or making some money, a little bit at a time, growing slowly over time.