Because many people come to Steemit to earn and when they earn very little or nothing they move on, because lets face it the content on Steemit is not all that great for the most part to keep people coming back to consume more.
I myself personally prefer the cryptocurrency platform ADZbuzz for earning and as a content discovery platform to follow my favourite publishers.
I think it is going to go mainstream once it is optimised and promoted in 2018.
RE: How Many Total Accounts Are There on Steemit? Active Users?