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.
The problem is that the wyatt interference came out of nowhere. It had no build up, there was no interaction or kayfabe connection between wyatt and ambrose.
When Kane interfered with the very first Hell in A Cell match, it was his debut - we had heard Paul Bearer mention the Undertaker's brother Kane, and that the Undertaker had burned him in a fire long ago, and that Kane was still alive, in the months leading up to Hell in A Cell. There was a connection between Kane and Undertaker, a feud which was the justification for Kane appearing and interfering in the match.
With Bray Wyatt interfering with Ambrose? It's the other way around. Bray Wyatt interfering in the match is the justification for the feud. You don't book a feud around matches (unless the feud is literally about a controversial match result, or evenly matched guys trying to one up each other in the ring - see Benoit vs Booker T best of 7), you book matches around a feud.