It is just a guess, but it sounds very reasonable to me that the admissions offices picks the very best and most promising candidates as soon as possible after they handed in their application since it is natural that the competition among other universities for those candidates is high and most of them have applied elsewhere, especially since no one would apply exlusively for Cambridge only.
Waiting an extremely long time sounds rather like you're in a pool of already rejected candidates who will receive their rejection jointly after the overall admission process has ended.
At least that's it what I believe in my case - I send my application by the end of December and haven't heard something until today (under consideration) and if they really had interest in my person they would have definitely told me that in the meantime.