If - and this is pure speculation for the fun of it - Larson's original intent had been for the bug people to be the major menace, why change it? Did ABC demand Star Wars-ish robots? Or was it just that creating an army of convincing insectoids was too difficult/expensive?
The Ovions we saw in the pilot film were pretty remarkable - some of the best alien costumes I've ever seen on television - but then, they didn't really do much more than sit and stand around (and play a harp).
I know that I'm probably just reading way too much into it.... but it's fun to speculate about this sort of stuff.