Welll.......
....it's a bit of everything from my experience living in the UK.
The majority of UK homes are smaller, and dare I say often much older, than their Nth American equivalents. It is not common to have a cellar or separate laundry room, particularly in homes built before say, 1980 (and yes, I did just pluck a date....correct me if I'm wrong please).
As a result, most machines needed to be usable as an additional work surface in the days before fully integrated kitchens...Twin tubs, small top-loaders, single tub wringers (where the wringers folded into the tub for storage) and front loaders could all be had with work surfaces. They also needed to be able to be moved to the kitchen sink for use.
Moving into the latter decades of the last century, fitted kitchens became more common which enabled a more integrated machine. As people pulled out old cabinets and put in new kitchens, they made allowances to have a dedicated washing machine space near the sink - this gave access to the hot/cold water and waste pipes. It also meant not having to wheel machines to the sink and helped free up space in the kitchen.
Front loaders are the only machines that are suitable for building under a benchtop....so after many years of running different types of machine, UK manufacturers (and now retailers) defaulted to building (though no longer built in the UK) and selling front load machines with very rare exceptions to this.
As to the size of machines, this seems to have almost sorted itself out. The vast majority of front load machines sold in the UK and Europe over the past 40years have restricted themselves to a fraction under 60cm/2 feet. It is a common and convienient size that works well with washing machines, dryers (also often in a kitchen) and dishwashers with most kitchen cupboards and drawer units being available in 30cm, 45cm and 60cm sizes (1, 1 1/2 and 2 feet).
Door size is also limited to overall machine size. You can put a bigger door in a 27" machine than you can a 24" machine and still retain the same structural rigidity....also, the drums of Nth American machines are larger in diameter and therefore volume.
Hope this all makes sense. UK and European guys, please feel free to correct me.
On another note though, Australia has also accepted the UK/European standard for kitchens....but most Australian homes have a laundry room. However, as people renovate, there is a move to a smoother, more integrated look in the laundry...so often appliances are no being built under the counter.....which is, along with water restrictions and a call to be more enviro friendly, causing a dramatic increase in front loader purchase.