I once had to go through 4 interviews (with 1 interview left) and at each interview I felt I aced it. But the last interview didn't come for 3 weeks (considering the turnaround time for the interviews before used to be 1-2 days at most).

The last interview finally came and I got the job. When I got the job I asked my boss why it took so long for the last interview and he told me it was because they were considering someone else but due to contract issues the person didn't take it up and I was the second choice.

Point being, the reason could be for anything. I would just drop a text/email to the HR or the point of contact and see what they reply.