I’ve been wondering about this.
At first I thought it might be because there weren’t enough things in roller derby. So I made this diagram to illustrate it. 
Now, nevermind about the interesting suggestions this diagram is making, because as soon as I finished it, I realised that skates could be classed as a tool. Then I realised that running and swimming were sports and they didn’t have tools. Then I thought maybe the water could be considered an additional element, but that was quickly dismissed because air certainly can’t.
It occurred to me that sports perhaps could be divided into those that were more of a game, and those that were more of a race, and then those that were a performance, like dance or syncronised swimming, and roller derby would be the only game that didn’t have balls or tools unless you considered the skates themselves as tools, or are they just methods of motion, like a bicycle in a race?
So from what I can see, the actual sport of roller derby doesn’t have anything that makes it completely otherworldy and uncomprehensible. I guess it’s just unfamiliar.
Therefore, citizens of the world, when confronted with roller derby, please stop acting like it is bizarre. If you don’t understand, it just means you need to get to know it a bit better.
People generally don’t get Roller Derby...it is so different