I read a few things at the time of the iPad's announcement in which people compared the name to the simple short form "pad" for a something completely unrelated, and why the iPad was likened to that, while things such as notepads weren't.
What I think it comes down to is something like a notepad has a significant first part, note, and when we think of that, it's "note" that has more prominence in our minds than "pad" so it's a pad for taking notes. Similar with a legal pad. The "pad" part is less important what type of pad it is, and "pad" simply completes the name.
A launchpad is focused more on the launch than it is on the pad. It's a place to launch something, rather than a pad being launched.
But when it comes to the iPad, the "i" simply isn't significant enough, it's not a word or a concept in its own right. While there's "I" for ourselves, we know that's not what that "i" is, so it doesn't stand alone, basically leaving just a pad.