It has been announced since 2010 – from 2014 on five gait tests on oval tracks require a 250m track or a combined track (P-track). The reason to take this decision in 2010 is that the long side of a 200m track is too short (only 45.44m) to allow a proper transition into pace, showing racing pace and slowing down again in a decent, horse friendly way.
Organisers of non-WorldRanking events can use a 200m track in combination with a straight track next to the track (combined oval track, aka P-track, see FIPO 8.4 and 19.2). For WorldRanking events however a 250m track is required for all WorldRanking oval track classes as from 2014 on.
The reason not to implement this decision in 2010 already is to allow organisers to adapt their tracks.