The article says due to front end complexity. It's in the last sentence of the section with that header.

> Drupal also has this complexity issue which makes developing sites harder than it needs to be, and is why we didn’t consider that platform either. I’ve talked to other agencies who have decided to drop Drupal due to its complexity.

I am sure I’m biased as I’ve spent a lot of time in Drupal. There are certainly some oddities wrt Drupal’s frontend but dismissing it entirely on that seems silly. Especially when it does a decent job of ticking some of their other boxes (e.g accessibility, open source, headless tooling).

I'd suggest using Drupal with a static generation tool, headless with Next.JS or Gatsby.