I'd say it's because of Miami Vice.
Just a thought, but I wonder if it's because our country was founded by the gun that we're constantly experimenting and wanting to develop something bigger. Honestly, why on earth would someone want a S&W 500.
I'm just thinking out loud, but whether it be initial colonization or the drive west, guns eliminated obstacles for us. That and having a free government (at the time) that learned from the oppression in Europe that the citizen should be armed allowed us to toy with caliber. The countries in Europe at least up to the founding of the US were founded by the sword.
Just a complete guess, though. The government didn't regulate what we had for a very long time either, whereas many other governments do. I want to say Argentina limits the citizens caliber to .380 ACP and in the UK, citizens aren't allowed to have a handgun (I think) or they require some sort of special permit like Chicago (or wherever).