Wherefore art thou iPad?

Dr. Drang suggests some reasons why the iPad isn't selling better:

Finally, there’s the big problem: storage. How does someone on a iPad access all of the photos and music and video and other files that are part of the modern digital life that Apple wants us to lead? None of us can be post-PC until all of our stuff is where we can get at it without a PC. That there’s been no clean, obvious, and reliable solution to this problem is definitely Apple’s fault, and it’s kept the iPad from being a complete PC replacement.

Here's another reason:

Apple's behavior severely limits the types of apps that are available on iOS. Whether it is due to actual restrictions, or just due to fear on the part of developers, there are a lot of "safe" apps on iOS, but very few apps that try to break the mold of what people expect from their devices. You get a lot of games, podcast clients, todo lists, camera apps, text editors, things like these — but not a lot of stuff that colors outside of these lines.

None of these app types work substantially better on larger screens. In fact, there are very few apps on iOS that you really need an iPad for if you want to get the most out of them. There's Microsoft Office,¹ and iMovie — but even for these apps, it's not entirely clear to me that most people wouldn't be perfectly happy running them on an iPhone, particularly on a 6 or 6 Plus. There are some niche apps (audio apps, for example), but they are just that — niche apps.

Hence, there is very little reason to own an iPad if you already own an iPhone. Unfortunately, the primary target audience of iPads — people who are inside Apple's ecosystem — probably do already own an iPhone.

In short, if you have an iPhone, and you want a second, more powerful device, why would it be an iPad? There's almost nothing you can do on an iPad that you can't do on an iPhone. It's just as restricted as the iPhone, and as a result, can't differentiate itself from the iPhone. But at the same time, the iPad is less portable, and lacks the phone features of the iPhone.

While a lot of people might be okay with their phones running only a limited set of app types, the same does not seem to be true when it comes to other kinds of devices.

The iPad isn't selling better because Apple's own rules prevent it from being the truly compelling device that it could be.

Addendum

Ben Thompson:

The problem is that must-have apps are exactly what the iPad needs to become indispensable. (...) I can’t help but reminisce about what might have been had Apple harnessed the incredible developer enthusiasm for the iPad in 2010-2012. More than any other iOS device the iPad needed help to make it indispensable to everyone, but Apple famously doesn’t like depending on anyone. And now no one cares.