Naturally, free services are very popular products because everyone likes to save their money. And, they can be handy bits of software if you're not somebody that's likely to keep their VPN turned on all the time and just want it for occasional use for staying safe on public Wi-Fi. Oh, and if you don't mind ads...
For most people, though, free services provide a false economy. They tend to have limited servers in just a handful of locations, often restrict you to a single device and almost always have a limit on the amount data you can use per day or month.
Those data limits rule out using your VPN for streaming or torrenting, and if you want to keep your VPN running 24/7 for a permanent privacy layer, a no-fee VPN just isn't going to work.