Well, for one, AV software started to classify any website with its code as 'infected' with malware. Then adblockers blocked their script, and even today their website doesn't load properly with uBlock activated.
These are definitely important issues but they don't sound like show-stoppers. Many people use anti virus software and adblockers but then again many don't!
Additionally they (or some alternative) may have been able to improve the software and create a simple consent process to avoid being blacklisted.
I tried it on a couple of sites I have with ~1k users using pretty heavily. I made $1.50 USD over a couple of months, and most complained that it was being blocked/giving security warnings when visiting the site.
Not that I am making much from ads either, but it just really wasn't worth it. Affiliate revenue, while small on a grand scheme, was much more effective .
Even at the start their rates were fairly bad, price crashes + being blocked must have made it a lot worse. And few legit sites are going to use something that gets them branded as hosting malware, even if only some of their users use software detecting it.
Additionally they (or some alternative) may have been able to improve the software and create a simple consent process to avoid being blacklisted.