Complex sentence: The Colle System has become more frequent in amateur ranks in current tournament play than in Colle's day, but remained popular through the efforts of Belgian/American grandmaster George Koltanowski.
Split into simpler sentences: The Colle System has become more frequently played in amateur ranks than in current tournament play than was the case in Colle's day. The opening remained popular among amateur players after Colle's era through the efforts of Belgian/American grandmaster George Koltanowski.