Because it's policy.  :)

DNN follows a different development pattern than some open source projects in that DNN is open source but closed development.  There are many open source projects where the community can contribute directly, and those usually have nightly drops so the code can be tested.

DNN follows more closely a process similar to Microsoft's.  Alpha code is written and tested in house.  Beta code is released to a small set of beta testers.  Release candidate code is given wide distribution to more experienced operators and then the code is released to the world.  This process results in a rather decent set of testing and bug response, since in open distributions such as you want, 99% of those who download the code will never file a bug, and many bugs filed will be inadequate reports and can't be fixed.

There are good and bad points to both methods, and in the end they both work for the appropriate development crowd.

Jeff