A lot of it come from the first advantage mover and the fact that Bitcoin has one of the best distribution in cryptocurrency. That being said, as time pass Bitcoin will lose some of its dominance but nonetheless Bitcoin could possibly remain the cryptocurrency with the biggest market cap.

Steem is bringing something new and is bringing a lot of positive thing to the crypto-space .

I'd recommend you to read my post on Bitcoin and or my other post on how to invest in cryptocurrencies. These will help you a lot to understand some of my ideas on those subjects.