(To address sibling comment: If I were colorblind, I would lead with that in any conversation about syntax highlighting; I am not colorblind.)

To answer the question: it's a feeling, like lots of things in software development. I tried "no syntax highlighting", found that I liked it, and I no longer use syntax highlighting. To say "specifically" how it's "better"... I'm not even saying it's better. "I like no-syntax-highlighting" is the statement I'm making (which, when it comes to syntax highlighting, is a statement a lot of people have issues with). So, from my personal experience, I take issue with the statement that no-syntax-highlighting is making things "difficult for the sake of it".

Try this out for analogy: I ate Red Baron pizzas every Friday night for 15 years, then I heard about homemade pizza 10 years ago. I tried making homemade pizza. It was good! ("I tried it and liked it") Now I only eat homemade pizza on Fridays. How is homemade pizza specifically better? It's better because I like it more. That's all there is to it. It's a preference.

(For the analogy to work, you have to like or at some point have liked Red Baron frozen pizzas. I happen to like them... the analogy is flawed though, I admit!)

(Let me preempt criticism that I'm comparing Red Baron frozen pizzas to syntax highlighting. I am not. It's only about the preference, not the object of the preference.)