There are several reasons why people tend not to follow back on Steemit. I will try to touch on few of them that I was able to think of. Please feel free to comment and don't forget to upvote.
1- Passive following: if you follow someone without any interactions on their posts, chances are that person won't follow you back because he/she doesn't check his/her followers' list. In addition, there's a high probability that your follow-out -f -nowhere creeped them out.
2- Forgetting to upvote: it is always great to choose who to upvote to wisely so that the content is getting what it deserves. The problem here, however that with the availability of many alternatives to increasing the upvote and post interactions using Steemit bots and other, many of the good contents are left with few interactions. This makes new Steemians that are still raising to feel frustrated when they get followed with no interactions on their posts.
3- Having a weak profile: usually when decide to follow someone back on Steemit, you have to go to their page and follow them. On the way there, you get a glimpse of the Steemian profile. Having a good profile here can the factor contributing to the increase of followers.
Please don't forget to follow and upvote and comment any additional reasons you think are also helpful and may the followers be at your profile at on time :))