Simply because the next round of fixtures were due to be played the following week and Spurs had no space in their calendar. The final isn't due to be played until April so there's four months to reschedule the game to see who will lose to Spurs in the final.
They could've easily postponed it, as you say, the final is not till April.
Easier to just boot out little Leyton Orient than have a few more delayed games though.