For me, it's the lack of apps. If there was After Effects, Toon Boom, etc for iPad, I might be interested. Right now, it's just not the right tool for what I need.
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Genuinely interested: What do you use your iPad for that you can't do on a Mac?
Sure - here are a few.
1) Filling out contracts - a big part of my job is filling out contracts, which I use the Pencil for in PDF Expert.
2) Annotating documents - my wife had a big paper due last week two states away, and I helped edit it for her with the Pencil. It was so easy, and I could not have done it on the Mac.
3) Listening to music without headphones - the iPP speakers sound fantastic, and the few times when I still use my Mac I am annoyed that I can't listen to music with the same quality. I love listening to music while I work.
4) Having a large screen on a lightweight device - I was very skeptical of the 12.9" screen size, but it is one of the best things about the iPP, not only the size but the quality. My wife has the one-port MacBook, and the Retina screen is so much better than my MacBook Air, but still can't hold a candle to the iPP.
5) Working without wifi - having LTE has enabled me to work on iPad away from home in a way that my Mac simply can't.
6) Scanning - I use Scanbot to scan documents for work, which I can't do on a Mac.
7) Siri - I know this is coming to Mac in Sierra, but it has been a big part of my workflow on iPad.
8) Small footprint computer - I will be working on my iPad with Smart Keyboard in keyboard mode, and then take a break for lunch or a shower and fold it into display mode, easily fitting on the ledge in my bathroom or the kitchen counter while I make my meal. I never brought my Mac into the bathroom and rarely into the kitchen.
Many people use iPhone + Mac to pull off some of the tasks I listed, and I used to as well. But switching to iPad has made it easier. One of the unexpected benefits has been not mind-switching between two OSes. Most people in this forum use iPhone + Mac throughout their workday, and it's hard to describe, but there was a mental relief to only working in one OS throughout the day once I switched to iPad, occasionally still using my phone.
Most people here focus on some tasks that seem harder to them to do on the iPad, whereas working on the iPad has made me notice the tasks that are harder to on a Mac. That's why I said 2/3rds of things are easier on the Mac, and 2/3rds are easier on the iPad.