Is there a specific reason that the cyber-skull is only available as an obvious limb? If so I'd like to know what the logic behind it is.

I'm not a Dev, but I'll take a shot here:

Even the best "realistic" cyberlimbs are fake up close and always on touch. If they can't get a simple limb to work right in terms of "realism" how are they going to get all the hundreds of muscles that are part of the face to work properly?

Have you seen modern animatronic technology? In another 60 years I think they could do it. especially with access to myomers. I just think it's silly that you can buy (for a crapton of admittedly) a drone that can pass for human better than a person with a cyberskull can.

Also, they have a rough picture of what a cyberskull looks like in the SR4A BBB. It's not pretty.
When taken along with what Kerenshara said about facial muscles (hell,it takes 6-12 muscles just to SMILE), and that even some sort of facial prosthetic really wouldn't look all that real (at best like someone's entire face'd been botoxed to oblivion...), I cansee why it's onl obvious...

Oh, I am with you on the "why the drek not?!". But in that case, I want my delta-cyberarm to be indistinguishable to the touch and under a magnifying glass. If they can't make a cyberarm (with few "muscle shape" issues) look completely real and feel real, how are they going to pull it off on a face? The rules STATE EXPLICITLY the limbs won't pass up close.

There is no valid reason. For all intents and purposes, all it is is a replacement for your actual skull. Not your face, not your eyes, not your nose, not your lips, not your ears. Just metal in place of bone. Unlike real cyberlimbs, it's not repalcing anything that truly needs augmenting except, maybe, your jaw strength? I dunno. All you really get from it is armor (if you plate it, which should be where Dermal Sheathing-like penalties come in) and some storage space inside due to optimization.

Man and Machine (SR3) has rules for synthetic cyberskulls. They give a synthetic cyberskull a concealability of 10. And in the fluff for cyberskulls says that they just replace and reinforce parts of your skull, rather than lopping off the entire thing and putting your brain in a synthetic metal head.