There's 2 reasons.
1) It's hard to transition an app to 64-bit
2) iTunes probably won't benefit much from it.
Point 2 is especially important. What would apple achieve by releasing a 64 bit version of iTunes? Currently, when I run iTunes, it consumes about 70 MB of RAM. It would take a lot for iTunes to require more than 4GB of RAM, that 64 bit addressing would give it. And even if it could, I really wouldn't want iTunes consuming 4GB of RAM. Then you could increase the speed of MP3 (AAC) encoding if it was 64 bit. But this stuff is pretty quick already. I think in most cases, it's limited by the speed of the CD drive you are ripping from, and not the speed of your processor. Movies and stuff play back just fine without skipping.
So, all I have to say is that it would be a lot of work, with no benefit, and nobody would even notice. We'd be much better off asking for 64-bit apps that matter like MS Office (outlook especially), and Visual Studio.