Maybe they didn't leave until the past few weeks.
A lot of other arguments such as the whole "Imps just suck," "Reps play together," or whatever doesn't seem to hold water due to the simple fact that these imbalances don't seem to pervade down through the lower brackets. Lowbie brackets are FAR more competitive (and many would agree far more fun because of it). The only thing that fits the evidence I've seen is that, indeed, the better Imp PvPers took their skills and bolted, leaving only inexperienced players behind to play against the Republic best.