A lot of it is due to the fact that it's much easier to sell an end-user on a plugin to their existing site that requires little to no server configuration than a completely separate system.
Whether or not it makes sense as a WordPress add-on is not really taken into account by users, who just see it as easy to set up and manageable from the same dashboard as the rest of their site.
Whether or not it makes sense as a WordPress add-on is not really taken into account by users, who just see it as easy to set up and manageable from the same dashboard as the rest of their site.