I think on the European side of things
I'd be pretty safe to say it had nothing to do with energy conservation. I think front loaders became most popular here partly because they take up less room, but mostly because of the way they were marketed.
Bearing in mind until the 80s most European front loaders used more water than most U.S. toploaders, and possibly almost as much energy (remember 90% of the energy used in a washer is used in heating the water, the less hot water used the less energy the machine uses.)
I think I'm right in saying that by the time automatic machines started to become affordable here in the 60s, a top loaders were marketed as being old fashioned and less automatic, their similarity to wringer washers and to some extent twin tubs enforced this idea. Infact when we went to visit family in the U.S. in 1997, Mum looked at my Aunt's 1979 Kenmore and said "is it actually automatic?".
Detergents for front loaders were called "Automatic" in the same way they are now called "HE" in the U.S. I guess this would lead people to think of high suds detergents as being for non-automatic machines, putting toploaders in this category.
Until a few years ago most machines in the U.K. had a hot and cold fill, yet the heater was needed to obtain and maintain hotter temperatures. This is not exclusive to front loader laundry in Europe though, as most Top loaders, twin tubs and wringer washers had built in heaters, and failing that many households had seperate laundry boilers before the days of automatics.
Since the 80s (possibly earlier) the average temperatures people wash at here has gone down. Almost everyone I know washes almost everything at 40c, and with a good detergent and the correct cycle I personally see no reason why clothes would need hotter temperatures than this. Detergents may be pushing for people to wash at 30c or even lower, but I know few people who actually believe clothes will be cleaned below 40c, and are unwilling to give it a go.
So basically I think while the reason for front loaders becoming more popular in the U.S. now is purely down to energy efficiency, this really didn't play a part in it in Europe.
Hope this helps!
Matt