It is not the software function that is difficult or time consuming.
Rather it is the decision behind taking the action in the first place.
There are a lot of factors to consider when making a decision like that.
Example:
There are a limited number of stickys you can have in a forum before it becomes untenable with sticky overload.
So it is not as simple as just granting the request of everyone that wants a sticky thread, It usually means that one or more must be removed, and if that thread is granted a sticky what similar threads would then also want it and could we accommodate them, and how do they stack up to the existing ones ...
So Reviewing all the current Sticky threads, and all the potential new ones, is a rather time consuming process, and a number of factors must be weighed.
Hopefully that helps to shed some light on what is involved and why it may take more time than everyone would like.