That would probably be because let's face it, invading someone without being summoned leaves little rewards. You get a token which you can use to invade again, and a wee little bit of xp. And that's it. You've got to buy all your stuff in order to invade folks. So, let's say you invade someone and get killed, that was a wasted token, and you gotta buy another. And that shit's expensive. The only reason there is to go out and invade someone is because you want to. That's it. You literally get nothing in return for it. And, as a result, the covenant that was focused on counter invading sits in a corner gathering dust, because guess what? Way of the Meek is a pretty shoddy covenant. You don't get anything from it. It would be cool if they did something where you ranked up based on defenders and their victories, and gained more tools to influence your world to help defend yourself. But all you get is a ring. And that's it. A covenant that needs two covenants to function was kind of a silly choice made by From.