Touché. But I don't see it as failure of either side; rather as using the strengths of both systems in unison.
The iPad(OS) is optimized for content consumption, is very portable, but has a restrictive environment that doesn't allow some of the low-level features required by _some_ professionals and hobbyists. While Linux is optimized for the latter, and though there are obviously good quality hardware and software that runs it, I like the iPad form factor.
Do I wish the iPad or Linux was more usable for my particular use case, so that I could just use one or the other? Sure. But I think that would satisfy a very niche intersection of users, so neither side has much incentive to build such a product. I'm more hopeful of a Linux tablet being good than Apple ever budging though. Some future iteration of the PineTab might be such a device. The JingPad also looks interesting.
