*Because environmentalist, economist, socialist, educator, campaigner, activist, etc, etc, are all too narrow.
There is no word to describe people who have interests across the broad field of sustainability. And there are many of us. 'Sustainabilityist' is clumsy (quite fitting for me) and probably an SEO disaster, but I'll stick with it until the linguists come up with something better.
Meanwhile, I've got a 'biography' section, this is what a Sustainabilityist looks like.