Why a certain period of time is needed for promotion of qualified applicants?
From my experience and what I have observed, the time frame is time for the academic to "earn their stripes", so to speak.

Some more factors that I have noticed that are considered, that do take some time to notice, namely:


The person's character - how well do they work with colleagues and/or students.
How the person performs under pressure and during 'down' times.
How much initiative the academic shows can only be observed over a period of time.


note: the above is not usually formally documented, but are important in any profession for someone going for promotion.

Also, as the university will be making a considerable investment in the academic, it would seem that they would like to see that the academic is willing to make a considerable investment (time, expertise etc) in return.