Why, because you say so?
Why did Apple make Safari closed source but the LGPL licensed Webkit open source?
One answer might be that Apple has no choice for Webkit but to remain open source thanks to the LGPL and its "ancestor" KHTML but it chose some proprietary license for Safari to "protect" itself from competition stealing Apples precious work.
Now your answer :)
https://plus.google.com/u/0/103674611711666213369/posts/7j1c...
What do you think what is the outcome when the overall open source software used nearly halved?
I leave this as an exercise to the reader (Hint: the last version of Mac OS X contains less open source software then every version before, overall and in licenses).