This is so developers can continue to test their software with the latest operating system available. Consumers really don’t need updates that frequently.
Want to join the OS X Beta Program? You can still sign up by following this link.
Both iOS 8 and OS X Yosemite are expected to be released to the public this fall. They might not be released at the same time, however.
See also: Apple unveils a beautiful new iTunes 12 design for OS X Yosemite, Apple's OS X Yosemite Is Already More Popular Than OS X Mavericks, and Should you upgrade to Apple's iOS 8 beta 4 or OS X Yosemite developer preview 4?