The reason for that is that demo programming is the coding equivalent of watch making: extremely precise, technical work that takes deep knowledge and long hours to achieve something notable. Knowing your tools and platform inside out is a necessity - and the more modern your CPU/GPU are, the less realistic that is. When I was writing assembly for my 8MHz z80, I could easily hold the full instruction set + CPU register layout etc in working memory. When you're working on a modern i7 + GTX780, that's pretty much impossible, and you have to resort to abstraction layers and tools that inevitably lead to the situation described by the author of the article. It's a classic case of extreme restraints working in the favor of creativity.
Interestingly enough, my few friends who are still into coding demos don't do it on modern architectures - they're mostly sticking to the C64.