I think it is mainly due to marketing. Social media companies like Facebook and Twitter do all sort of marketing. On top of that, they do so much advertising. In a way, even though people pay them for the ads, the scale of the companies that advertise with them is , in return advertising for the Facebook's and Twitter's of the world.
To go further, the backbone of Steemit is its users. Considering the decentralized nature of the platform it is up to the users to do the marketing and on-board new users.
If I were to pick one big reason it would be the advertising piece. Since you can't buy ads on Steemit, big company's are going to use Facebook and Twitter where they can attract more people to their various companies. That is also part of the beauty of Steemit. The fact that the experience is all about the individual and at the same time, the community.