The main reason is spoilage.  That is, we want you to be able to enjoy the final game and the difference between beta and final is significant.

The second reason is to decrease the signal to noise ratio.  Most beta reports aren't very useful because they are incredibly vague "I got a CTD!", lack any sort of hardware or software info, etc.

That said, we are still adding people to the gamma testing as long as they can:

a) Make sure they are available on IRC quite a bit.

ARe technically proficient enough to self-debug some things.

c) Remember to make reports that include hardware and software info.

d) Know how to use Smart Exception and include debug.err