I wonder, what is taking him so long?

He isn't a starter at his team anymore and not much time left for the end of the transfer window for European leagues.
Currently his contract still has about 2 seasons left at City and seeing from some of the statements given by Akanji it seems that he still wants to stay at the club, it's just that obviously this is difficult because Pep's progress towards young players is currently very large so for old players now it will obviously be very difficult to survive including Akanji and Gundogan who seem to be subtly expelled now.
Akanji is actually still very progressive to be at another big club now especially seeing the position he has actually still very possible to play in several positions so it will be an advantage if other clubs bring Akanji especially if you see the duration of the remaining time before the transfer season closes then it is clear that this will be a fast negotiation for now depending on who dares to make a better offer to City as a clause release.