Why is commercial sandwich bread so popular in the US and UK?
To put it bluntly, convenience, price and availability. While more artisan or rustic breads are available in supermarkets (where the vast majority of westerners shop, especially the US and UK), they are generally more expensive, a less convenient shape and size for sandwiches, and don't keep as well.

Having said that, in recent years in the countries mentioned, there has been an increasing interest in home baking of all kinds, including bread.

It's worth pointing out that some western countries like France and Italy still have a strong tradition of buying fresh bread daily.