The reason for the secrecy has been explained many times as being mainly to prevent abuse such as sandbagging (manipulating scores to stay in a lower tier with easier playing conditions).
There are several other reasons as well (I surmise, at least). One is that the figures may be adjusted over time. Several months ago the whole system for figuring averages was changed so that after a certain number of ranked rounds at a given tier the average does not go up but only goes down (when the player shoots a round that is better than one of his/her existing "best" rounds). Now, when a change like that is made, and the whole meaning of the average changes, it seems obvious that WGT might also want to adjust the average required to advance tiers - keeping the same numbers would be like comparing apples and oranges.
There is also an element of experience because you are supposed to meet certain targets not only in terms of the average but also in the rounds played at your current tier. (This figure is also secret.)
Plus, the whole set of tiers is still open to revision - WGT has suggested that more tiers could still be created. There was a big shakeup in September 2010 when several new tiers were created, and obviously the figures for advancing were all changed.
If they published these figures, players might feel "entitled" to advance (or not advance!) even though WGT feels a need to adjust the targets somewhat.
Secrecy notwithstanding, players have calculated, based on their own experience and the experience of others, the average currently required to advance - not officially, but with pretty exact certainty (at that given time). You can find these by searching the forums.
Finally, as the great Yancy would say, "This is all just beta, folks." What beta means in this day and age is unclear - folks have been playing this game in beta for years - but beta it is.