To be honest, a big part of it is likely because every other publication also uses ads for monetisation, and users have been trained to expect content for free

To be honest, a big part of it is likely because every other publication also uses ads for monetisation, and users have been trained to expect content for free with ads. So you then have an issue; if you charge for your work in some other way, and people can find roughly the same information elsewhere on an ad supported website... Then most of them will go with the ad supported free option instead. Likely enough of them in fact that your work isn't sustainable. It's the same issue as with mobile apps and games; because most of those are 'free' and supported by ads and microtransactions, consumers have come to expect all apps to be free and supported by ads and microtransactions. Anyone who tries to actually build a paying audience finds its not enough to make a difference, like Nintendo did with Super Mario Run. To add to this, it's also likely because the level of competition is so high, and free options are so high quality. For 99% of topics you could originally cover in a paid for magazine or book, there are wikis, volunteer run websites and YouTube channels covering said topic. To justify a paid publication about a topic like this, you'd have to do better than them in a meaningful way, which is not exactly an easy task.