there's a lot of factor that is affecting the delay. for example once an update is released by google it will be modified by let say samsung to match its device, then they will need to modify it for snap dragon device and exynos, next they will need to modify it by country to match regulatory standards, if your phone was issue via a postpaid plan it will be program a bit to match that provider needs. something like that.
The applications these developers make are not guaranteed to work without encountering problems along the way. The late arrival of updates hampers the developers' ability to make any changes to their applications, and even put the user's safety at risk.