Or it could be:
They wouldn't promise us to play him / accept any costs associated with not playing him
Maybe their game style wasn't suited to him or to us
Maybe Allan had an issue with the club or the city
Or we the club had problems with them or people at their club
Allan grew into a role as a "nice-to-have" squad player, especially useful when other players got injured (bytheway, i didn't notice problems with Hertha's game style/tactics, apart from the mistakes many young players make), until he went to the U20 Copa America in January despite Dardai asking him to stay in Berlin and warning him that he would lose his place in the matchday squad.
He came back slightly injured and it seems Dardai lost faith in him, he barely even played during the second half of the season. Whenever he played, he looked good going forward, he played good passes, had a good range as well. He is very weak in defense, though, which would be an issue in the Premier League just like it was one in the Bundesliga. He had a habit of getting caught out of position (mentioned it: mistakes many young players make) as well and according to some rumours (only rumours though), he didn't take it well when he was criticized in training especially when it wasn't the coaches but one of the more experienced players.
Allan stated several times that he wanted to stay in Berlin, so apparently he didn't have an issue with the club or city.
I think the main problem really was, that Hertha only saw him as an additional squad option and that it doesn't make sense to loan him to them if he can't get a work permit.
Now i suspect, that the problem is, that no club, that plays in a strong league and in a european cup will make any promises about playing time neither to Klopp nor to the player. In my opinion, he's just not good enough yet to play an important role in such a club (now that i think about it, maybe Freiburg if they qualify?)
He is talented though, Klopp saw it, everyone can see it, that's why i hope he can become a good player, even though i'm afraid it might not be in England.