I do mobile development, mostly iPad/iPhone development full time for a product development company (see profile if you want to email for a quote).

Facebook for iPhone was originally written by a non-facebook employee from what I understand who's now left the iOS development environment over philosophical differences with the app approval process. While facebook publishes their own app now, again, they weren't exactly the first to hit the platform in the first place.

What exactly are you expecting skype to do differently on the iPad than the iPhone? What are you expecting the non-iPhone shaped UI to give you that the iPhone shaped UI doesn't? The iPad isn't exactly a perfect platform for videoconferencing (no camera), and the phone version works fine.

All that said, lets talk about technical/policy/business issues which cause people to not specialize on the iPad.

First off: Business reasons.

Business wise, the iPad has very few users compared to the iPhone/iPod Touch formfactor. So when you write to that audience, you're paying for a generally more expensive app, to reach fewer people. Is this a bad thing? For companies that could grow too fast, it might be a positive thing, but on the whole a smaller audience is bad. On the plus side however, those who own iPads tend to be more affluent and are willing to buy more expensive applications, so you can get higher unit prices to somewhat compensate for that, if your concept is appealing to people.

Next up: Policy Issues

Apple is not the easiest of companies to "write software for". And while you're not writing it "for" them per se, you have to jump through a series of hoops they tell you to. And with the iPad, you have a list of guidelines to follow which are quite a bit different than with the iPhone/iPodTouch sized apps. Your apps need to handle all orientations for the most part, your apps need to observe the more complex menu-ing paradigms, your apps need to usually have more sophisticated art assets, and you also have a higher expectation of not looking "cruddy" for lack of a better word.

While the enforcement of these "guidelines" are all over the map, people routinely DO get rejected for violating some or all of them. This means the cost of iPad development goes up.

Technical considerations:

Hybrid apps, those which work both on the phone and pad in the "native" resolution of the platforms require you to either A> violate the assumptions of one or both platforms or B> create completely different screen layouts for each "mode" of the application. Those "modes" would be iPhone, iPad landscape, iPad Portrait. Menuing is different in the two orientations on the iPad, and expectations of pretty animations are much higher in general.

Until November or so, you'll see that additionally that iPad is on 3.2.1 of iOS and iPhones are on 4.0.1. This means the iPad is missing many of the cutting edge features that 4.x has. So if you're writing a hybrid app, you have to work around sometimes having certain features, and sometimes not, and additionally you have all the issue with different aspect ratios, etc.

Hopefully I've explained some of the issues. I've tried to be brief, this is a complicated subject, but basically, this is it, and it's also a huge barrier to entry, which means many good business opportunities still exist on the iPad.