Don't you think having so many competitors offering free notes apps is a large part of it?
There's work arounds (for example I forgot the name, but one plugin allows you to setup onedrive to share just one folder via logging in on each device), but the syncing story wasn't great. Quite a bit of extra setup across all devices, more points of failure etc.
Hence the smart business model, of let us handle it for you for a small cost.