Earth 2 was NBC, not Fox.
The issue with the rpg had to do with licensing. Fox owns the rights to Firefly (not entirely sure what it all entails.) Joss Wheedon owns some other rights. He made Serenity for Universal. The RPG was licensed from Universal. It just doesn't make business sense for Fox to allow an RPG to use its property without compensation especially when Universal is making money off of it. I don't really think it matters for the RPG.
Maybe now that MWP doesn't have the license for Serenity, Fox will allow someone to make a Firefly rpg.
My bad, NBC was the other network SciFi shows need to avoid.
Fox killed off Firefly, John Doe, Dollhouse, Werewolf, Brimstone, Millineum, Terminator, and also Tru Calling.
NBC killed Earth 2, The Original Star Trek, and The Bionic Woman.
There were other shows that were cancelled by both networks, as well as some others, that I don't ncessary think of as SciFi (though others might): The Tick, Jeremiah, Jericho, The Lone Gunmen, etc. I'm probably forgetting a lot more as well.
As for Fox and compensation, it was my understanding that they were approached and had the chance to do a license deal from MWP, and turned them down. Could be their rivalry with Universal, but they did have the chance to make money off of it.