If you focus on 'diversity', you miss the bigger picture, because of course every place is unique. The US has for example far more pronounced extremes of urban vs rural (which many too forget). The real issue is distance, and how Europeans and Americans in general have vastly different concepts of what's far.
If you're in the US or Canada, driving for 10 hours means you're going to visit your aunt. Doing the same anywhere in Europe means you're likely 2 countries over, and it becomes a 'huge trip' in people's heads. Heck, I know several people who have driven more than halfway across Canada, in Europe no-one who drove e.g. from Paris to Istanbul.
And while the European market is supposedly unified, in practice there are significant variations in pricing and availability as soon as you cross a border, simply because that's something few people do. And of course the language difference means most local media is opaque to outsiders.
So I would say that the US is definitely diverse, but it's a gradual diversity, that transitions smoothly from ultra-urban to no-one-for-miles rural. It works on very large distances, and is mediated by a shared media, politics and language. European countries meanwhile are much more homogenous on the inside, but there are a lot of forces keeping each one unique.