Marketing basically. They wanted the console to look more like video equipment and less like a toy. This concern was because of the video game crash of '83
The PAL SNES just reused the curved design of the original Japanese Super Famicom. I also find it way more appealing than the US version, but I grew up in Europe, so it is the one that I saw back in the day.
I'm the opposite. Grew up in Australia (with a 64, to be fair), love the US design. Our ones can't be stacked, and are missing top labels. A collector's nightmare!
US market was huge and US subsidiairies could impose their very own vision to HQ. It’s been the case for a lot of vg companies in those days. Check Capcom. Plus everything must look badass in the states, especially as the Genesis (Megadrive) was king back then there and looked way more mature.