Why finish it when you can do another 3 or 6 month feud later? Good place to stop it and keep it hot for later. Unfinished business. Both guys looked great coming out of it, and so did Wyatt. Relevant again.
It's not hot, though, the feud has lost a ton of steam in the last month (mainly because it's been five months since the Shield breakup and Ambrose is still looking for revenge and not getting it). It was the right time, and right build for Ambrose to beat Rollins there. Plus there's no reason you can't end this particular feud, because their rivalry isn't over and won't be over probably for their whole WWE careers. You can have Ambrose get his win over Rollins now and return to the rivalry later. The feud ending doesn't mean the rivalry is over.
But for it to be nearly a year on from the Shield ending (if this is picked back up circa WM) and Ambrose STILL hasn't gotten his revenge on Rollins, or beaten him? This is a career level rivalry, there's no need to drag out the initial reason for the feud forever and keep denying any victory to Ambrose. You're not going to be able to sell the initial rage/fury from Ambrose getting betrayed 10 or 11 months on. By then it should be something different.
I mean, how many times has Cena beaten Orton in a feud? Four? Five? Now that rivalry is total ass, but it is an example. Austin won three separate feuds with the Rock during their time on top, but they remained rivals throughout, with Rock winning the fourth one.