Because BW was on the horizon so they had to rush the league. Like the BW games and the BW anime needed to launch at the same time, and DP was getting long, so that resulted in DP's league being short, and Tobias being created so they wouldn't have to write a battle that isn't all DEM.
I don't think that's the case, because unlike us fans the writers for the show would have known about the BW games months before we knew about the new Gen (which would amount to almost more than a 1 and half years worth of time in total).
Them knowing it early is also necessary because unless they know about it before hand, they won't be able to plan out how the series would go, like who would be Ash's new companion for the new saga or what would be the base plot for series, etc.
There is also the fact that on average they make episodes 3 months in advance (that's what i heard during the time of DP Saga). Which means the planning process for an episode would most likely start a few months before that (i would take a guess and say that it would take around a one months time for them to make an episode, taking all points into account i.e., from the conception of the idea for an episode to writing the story and then animating it).
So, i don't think they rushed the DP Saga in anyway especially considering the gap we had between Ash's 7th Gym battle and his 8th Gym battle.
Instead i would say that the long gap that was there was because they wanted to fill in some episodes so that they could match the dates and start the new series on time.