I’m not saying it should be illegal to release games for only one console. Obviously not every studio is going to have the bandwidth to develop for every platform, and some games will use special features of some systems.
What I’m saying is that it should be illegal for console makers to give any special incentives or preference to developers to do so artificially.
You have to start somewhere. Most people are too scared of corporate retaliation, so it’s a situation where no-one wants to jump first and be left out in the cold if everyone else chickens out.
In that sort of scenario, it takes one group deciding to take the risk that can help tip the next domino.