If you are looking for simple answer, I can tell you this, it is going down because it is not going up!

Now moving to second part of you question, for this please open coinmarket website and check altcoins prices. If you don't see anything I will explain. Almost all alt coins are in red, which means they are going down too. So now I hope you can see with your own eyes one of the effect in crypto world that BTC price going down has.