The reason for Shatter being so heavily played - and I've said this before - is because Shatter was, and still currently is the newest PvP mode. Before 4.0, any older modes were typically rendered obsolete, especially Secure and Slaughter, which were left at 50. Seize was the first lv 60, and still arguably the best Frontlines mode, but with all the new blood that came in around 3.5 and now in 4.0, Shatter pops frequently, and involves less direct combat between teams, thus its more favorable to newer or lesser skilled players. With little/no knowledge of older modes, and no active queue for it, these new players come to think that Shatter is all there is.