Shake it but don't break it, Emil. I'm kinda surprised you posted a tune that features a banjo so prominently, a tenor banjo anyway - yot-digga-digga-digga-digga...
I think the one-word answer to the question - why the electric guitar? - is: AMPLIFICATION. To play to a big crowd ya gotta play loud.
Here's one of the two inventers of rock-n-roll playing a big crowd:
Bo Diddley, Road Runner
http://youtu.be/WOOFx9c6qyA
Anyway, great dance tune, Emil. Flappers, like rockers, broke all the rules.
Hey, here's a mish-mosh of Jazz, Big Band, and Rock-n-roll:
Brian Setzer covering Louis Prima's Jump Jive and Wail:
http://youtu.be/aHWcN5YxuYc
Just a tad more choreographed than the folks at the race track, but still good, eh?