Because for many (including me) it's by far the best one. I get a system-wide adblocker and vpn on my phone and computer with really good apps. Nothing else has been both so good and easy so I'm more than happy to pay for it, it's also only $30 per year. I really like their telegram group too, they post news and articles both from them and others about anything related to privacy and so on. They recently wrote about Microsofts addition of ads in Windows and released an update that disabled those.