I will simply echo what everyone has said...
That many UK houses wern't bulit with utility rooms hundreds of years ago. It's also I think convenience, as many kitchens are located next to the garden door, so if it's a sunny day you'd take the washing out and hang it to dry. I think US users tumble dry all year long, with many of us using it mostly during the rainy weather. (That being said, we don't have the best of weather either!)
Of course we still have top loading machines but they're not very common. I reckon that due to us liking to have our washing machines in the kitchen, manufacturers have taken a hint and instead made large capacity machines like the 11kg LG steam and Hotpoint Aqualtis to fit standard kitchen spaces so they fit in the kitchen. It's mostly common to see just a washing machine installed in a kitchen with no dryer, sometimes the washing machine is placed next to the dishwasher. People with utility rooms have washers and dryers, or even have both in the kitchen too.
I think back in the 60's or 70's people preferred twin tubs and toploaders, but gradually everyone has grown to accept front loaders, which is probably why they are so common here.