Licensing most likely. Apple's runtime is under their own custom license (APSL) which is GPL-incompatible.

I was about to correct you, but, in looking over the FSF opinion on APSL 2.0, I noticed that while APSL 2.0 is considered a free software license, I had forgotten that the APSL 2.0 license is still incompatible with GPL.

http://www.gnu.org/philosophy/apsl.html

Huh. I was pleasantly surprised by the sentence: "Apple's lawyers worked with the FSF to produce a license that could qualify."

Yes. RMS was highly critical of APSL 1.0

http://www.linuxtoday.com/mailprint.php3?action=pv&ltsn=...

So Apple revised the APSL to address some of his concerns.