I don't think it's that complicated. The iPad has been primarily a consumer product up to this point. It's great for reading things, web surfing, and even light-duty office stuff. It has not, however, been particularly useful for creative and design work, because the UI was designed for Steve Jobs' finger. A finger or passive stylus isn't accurate enough for professional graphics or design. The Pencil fixes that. Ba da bing, you have a "Pro" device. Someone who does that kind of work professionally now has an iPad available that he or she can, with great precision, draw on. The consumer product is the iPad mini or Air. The Pro product is the iPad Pro. It's not that complicated.
[doublepost=1480360423][/doublepost]
You could say that about any 'pro' feature, that it should just be available across the entire line. For consumer-level iPads, I think Steve Jobs was right about getting rid of the once-ubiquitous stylus (Palm Pilot, anyone?). People who just want to read and click (mostly) all have fingers and can do without carrying around a stick they're going to lose.
The internal iPad hardware required to work with the pencil adds extra cost to the device. Consumers who are just going to read and poke don't need that hardware or that cost. Graphics and design professionals do. That's what makes the case for having a separate 'pro' line.