Because Apple doesn't have to test their OS on a variety of hardware not in control by them, Apple can keep a close lid on the development.
Microsoft has to release Betas to the public really.... that's the only way to get feedback on the infinite hardware configs of PC's out there.
The only real public release of a Beta of Mac OS X, was the Mac OS X "Public Beta" (Kodiak) way back in September 2000.
For the rest all betas were developer builds.
TBO all X.X.0 releases are pretty buggy. Not all developers are ready, and some hardware drivers are not finalized, and Apple needs the massive user-experince feedback after the new Cat has been released.
For home use: Get it right away!
For pro-use: wait for the X.X.2 update...