There are lots of reasons why Drupal was forked to create Backdrop. Of those, these are the most notable.
1) Technical Difference
Drupal 8 is a very different piece of software from Drupal 7. Though many of the features in Drupal 8 are nearly identical to those in Drupal 7, the code underneath bears very little resemblance to the Drupal of the past. We see value in maintaining the code that has a proven track record of success. Is it perfect the way it was? Of course not, that's why this is a fork, and not long-term-support. Backdrop CMS will continue it's track record of adding new features, and will also proceed with modernizations and other improvements as deemed necessary by the community.
2) Diverging principles
The issues that are most important to the Backdrop community will not be the most important issues to the Drupal community. When projects' principles diverge, so will the code. See our principles for a detailed look at what Backdrop stands for.
3) Target Audience
Backdrop CMS is specifically intended for use by non-profit organizations, educational institutions, and small to medium sized businesses. Backdrop best serves these kinds of organizations: ones that need the complex functionality of Drupal 7, but without the expensive upkeep. As Backdrop CMS changes and improves, it is the needs of these organizations that will be our highest priority.
4) Decision Making Process
How did Drupal 8 end up so different from Drupal 7? It probably has something to do with how decisions are made organically for the project. If we disagree with the outcome of that process, we need to choose a different process for decision making. See our leadership structure.