Dortmund missed out on being in their best since the start of the season and they only started playing well in the competition when they were eliminated from the champions league, maybe that was what took their whole attention and motivation in the Bundesliga this season.
They managed to compete in the UCL and domestic league at the same time for years before, so it's unacceptable if they can only play well in one competition over the others IMO. Well, hopefully they can learn, especially the board, so they can make better decisions. I don't remember where I read this, but I heard Sahin was hired because the board feels he understands the culture and whatnot[1]. While the emotional aspect of football is important, they do need other things to back it up. Hoping someone to be another Jurgen Klopp is a bit unrealistic, too. CMIIW.
[1]
https://www.skysports.com/football/news/11899/13294030/borussia-dortmund-sack-nuri-sahin-what-went-wrong-do-bundesliga-giants-really-need-a-dortmund-guy