The reasons are simple as they are complicated.

First and foremost being that Bitcoin is the oldest crypto that there is. Is we trust the anti-fragile nature of this phenomenon then that would make the bitcoin the most resilient crypto in existence. 

Then there is the Lindy effect to be considered (Def:  a concept that the future life expectancy of  some non-perishable things like a technology or an idea is proportional  to their current age, so that every additional period of survival  implies a longer remaining life expectancy)

Then we also need to keep in mind that Bitcoin is also the market leader. Thus the crypto community does try make sure that Bitcoin is in a good position because its position affect's all coins.

Steem on the other hand enjoy these perks to a much lesser extent. That being said, we need to keep in mind that steem token unlike many others has a real world use. This is an unbelievable advantage over other coins.