"Holding back" is an integral part of life in general.
When you meet someone for the first time, do you tell them everything about yourself? When I meet people, I don't say "Hi, I'm Jason, and I'm an atheist!" I don't say that because people might not respond well to it, I let them get a feel for me first, and then slowly divulge more of my personality.
In this case, it's simply good business sense, ESPECIALLY with a product like the iPad. No one could have known that it would be so successful. Had Apple thrown every bit of technology they had into it right away, their manufacturing costs would have went up, and if the device failed to sell as well as they were hoping, they just screwed themselves. So they put out a decent device to "test the waters" and then if it succeeds they upgrade it the next year with a few more things.
It's just good business.