I take a stab at this. If you ask me.... it is more of a trend then anything else. Back in the day of old school pattern. 2 strokes with pipes were king (in my book they still are). As the rules changed so did the engines/planes. The winning guys were using 4 strokes do to there better power curve compaired to the 2 strokes of that period. Thus, The winning combo is what everyone has to have.

In the 2 stroke world of engines, until recent years, engine companies (Webra,OS,etc..) are now making 2 strokes more " Pattern friendly" by todays standards. If/when some of the top notch pattern fliers start winning with these new 2 stroke combo's..... I predict a swing again of 4 stroke to 2 stroke......