How many factions are there?

Some people can’t grasp that more races mean a better game; they confuse alt-upgrades, wondering why they need even more races. Others have lost interest in the game – they’ve had enough. Some lack the imagination – they wear the same outfit for half their lives and struggle to come up with something new. But I’d be happy with 100 or even 200 races, as long as each one has completely new units, skills, unique heroes, and a backstory. Why are races from other worlds present in Warcraft 3 and WoW, but it’s considered too difficult to come up with them for Heroes 5?