#1) Voxels have been around for a while.
#2) Notch wasn't the first person to create a "cube" world, he merely got lucky and was the first to really popularize the idea.
#3) Notch borrowed the idea from Infiniminer and through feedback developed it into what we know as Minecraft today.
#4) If ideas were restricted to the "most popular" companies/people to use them (or most recognized), the world would have fewer things(especially when it comes to video games). Imagine if the first company to make a first person shooter was the only company allowed to make it, that would be fucking stupid.
#5) Cube World, for the most part, is nothing like Minecraft. It doesn't involve digging/building(at least in the way Minecraft implements it) it's mostly an RPG with experience, quests, etc etc.
#6) Notch hired one of the devs from Cube World to work on a "secret" project of he's, something tells me he isn't mad.
#7) Always end your points on an odd number, it makes people feel vierd.