I agree that Apple really needs to work on differentiating the iPad from the iPhone. The issue isn't the hardware, there only so much you can do with a slate, but software. For a while there the iPad wasn't just a big iPhone, but a big tablet. A new feeling device with different layouts and more productive uses compared to a 3.5" phone.
But things have changed. The iPhone has grown to 4.7" and 5.5" which greatly boosts their potential and usability while iOS has seemingly dropped iPad optimaztion (that music app and app switcher are a disgrace). Right now, for the first time, I'm starting to see the iPad as "just a big iOS device (iPhone)" instead of an iPad. Does that make sense? It started as a big iPhone, but grew into a new category and is now declining. I fear an iPad Pro becuase I don't want a permanently crippled 9.7" iPad.
Also, as others have stated, the iPhone is the flagship product of Apple and the "do everything device." It's your phone, your music player, your mobile internet device, your game player, or communication system, your camera, and so on. The iPad is not currently. But I don't agree with other posters claiming to understand why Apple emphasizes the iPhone as an excuse. Apple is more than capable of focusing on the iPad and the iPhone. There is no reason the give iPad the leftovers. Remember that for a time the iPads were the ones to get the new processors first.
Do I love the iPad? No, but I enjoy it. My iPhone is still my preferred device if I had to pick on, but I still think Apple needs to put more into the iPad.