I'm not sure whether this is accurate or not, but I've heard the theory that Prohibition is to "blame."
It goes something like this: when Prohibition ended, the brewing industry experienced rapid growth in the U.S., as breweries either started up from scratch or expanded their operations. Refrigeration technology had progressed by this point, so it was much more practical and cost-effective to make lagers than had been the case in the past. So there was a shift towards lagers, which are meant to be served colder than ales.
I really haven't noticed much of a difference in beer temperatures during my travels. What I have noticed is that cold, mass-produced lagers seems to be gaining in popularity everywhere: I was stunned to see how much Budweiser was being sold and consumed in Irish and British pubs. (I also saw a lot of "extra cold Guinness" taps.) This anecdotal experience would be consistent with the Prohibition theory, in that it suggests that people everywhere tend to like cold beers, and that the U.S. was just quicker to cater to that taste because it had to rebuild its brewing industry right around the time that it became easier to brew and serve cold beers.
My contrary data point, though, is water. American restaurants tend to bring you water that is half ice, while overseas it seems to be closer to room temperature.
