Because it is hard
There isn't anything revolutionary about Apple's offering in a technical sense. They are ideas that have existed for at least 30 years. On the other hand no-one, has brought them to commercial reality, so in that sense it is revolutionary. Most of computing is reinventing or rediscovering ideas that are from the 60's of 70's. Astoundingly little is actually new.
Apple is providing little more than named blocks and an abstraction over parallel-do. But the magic is in the manner in which blocks are scheduled. It avoids the kernel where possible - thus avoiding expensive system calls for synchronisation if it can, and it can use dynamically configured schedulers that are a best fit for the hardware you are running on. Part of the trick is also that by queueing the parallel-do operations you are implicitly providing some causality information, at a level that is more helpful to the system. Coding this with locks and threads would be very time consuming, and nightmare to make configurable to different hardware.
@Voodoo. The problem with this "best practice" is that this is too simplistic. Just protecting data objects with locks, monitors or the like, does not protect you from many other sorts of subtle bugs, and can lead to very poor performance. Sadly it is very easy to craft code that runs across many processors that runs no faster than the original code on a single processor.
The old idea that it is easy because you have one core to do each user task, and maybe one other is very naïve. It takes no notice of the future that will give us 16+ cores on a typical machine in fairly short order. Nor does it help with the grunt work of more advanced applications. Larrabee is next year.
And of course we get the usual trolling from Louis Savain, who keeps regailing everyone about how brilliant he is, and how he has solved it all. Without ever having turned a line of code or ever demonstrated even the simplest actual running examples of his world saving paradigm. Slideware of ideas had in the shower. Bad ideas too.