DC played great at the start of the season. They got some great wins. I don't know what happened to them at the end of the season. After the performance of the whole team broke down a lot. But they played really great against SRH. If they hadn't lost early wickets in the final, I think the final would have been competitive.
DC was one of my favorite teams, because they had the largest contingent of young Indian players, in Shreyas Iyer, Prithvi Shaw and Rishabh Pant. But they were unable to maintain the momentum. And in more than one occasion, they lost matches which they could have easily won. I would still say that they were unable to win the tournament, because the players cared more about their individual records.
You told that 'the player were unable to maintain the momentum. It is the main things. If you watch DC's 3 matches against MI, you will see that DC has lost 3 matches in a very bad way. MI is the only team in this IPL who are very stable and each player has done their respective responsibilities properly. The rhythm of all the other teams has fallen. Some matches played well, some matches played very badly. When playing against a strong team like MI, DC's players should have focused more on their own performance.