The game industry is not just coders, for one thing. Also worth noting this effort doesn’t preclude anyone from organizing a software developer’s union; in fact

The game industry is not just coders, for one thing. Also worth noting this effort doesn’t preclude anyone from organizing a software developer’s union; in fact some people are working on that goal, and it’s also possible to join IWW as a software developer. But to offer a different answer to “why just games” - this particular effort is newsworthy right now because it was organized to be promoted heavily at GDC, which I believe just ended.