Wound G strings for electric guitars do exist. They were more common in the 1940's, 50's and 60's, when jazz and folk were in vogue. Those styles of music didn't typically entail the bending of strings like it does now. Bigger strings were also a carryover from acoustic guitars due to the fact that heavier strings are acoustically louder.

In the 50's, a set of 11's were considered to be a light gauge.

Electric guitar began trending towards lighter gauges of strings in the mid-late 60's, when the perception of electric guitar being a 'lead' instrument was created. Couple that with the rising popularity of blues, bending strings became a lot more commonplace. Electric guitar became more dependent on the amplifier it was driving, to make it sound the way it was becoming more commonly perceived as sounding. So extremely heavy strings simply weren't needed anymore. And they were making the techniques that were becoming increasingly common on electric guitars more difficult.

Wound G's are still used by Jazz players and players that do a lot of downtuning for the extra tension they create. But they only exist as a minority now.
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Quote by Axelfox
Heh heh, he said G string. 

Seriously, I think it's about tone and volume. A big, thick wound string moves more air and is better for filling up a sound box. On an electric, moving air doesn't matter as much and a plain steel string gives you better bends and vibrato. 
Death to Ovation haters!
thanks for the info, people. i was hoping that the acoustic steel string, G string was not wound. 
begginer geetarest.