There are lots of reasons why Drupal was forked to create Backdrop. Of those, these are the most notable:
1) Technical Difference
Starting with version 8, Drupal took a very different turn from Drupal 7 and previous versions. Though many of the features in Drupal 8 are very similar 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 already includes many modernizations, and will continue adding new features every 4 months.
2) Diverging priorities
The most important issues for the Backdrop community are not necessarily the most important issues for the Drupal community, and vice versa. When the priorities diverge, so will the code.
For example: Both Drupal and Backdrop CMS have configuration management as a feature, but they are implemented differently for each project. Backdrop chose to store its configuration in json files, without a required database cache, because the json solution was more in line with the following 2 principles: Simplicity, and Speed & performance.
See our principles for a more detailed explanation of 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 organizations that need the complex functionality of Drupal 7, but without the expensive upkeep required of modern Drupal.
This doesn't mean that Backdrop won't also be a great product for other websites, but as Backdrop CMS changes and improves, it is the needs of these organizations that will be our highest priority.
4) Decision Making
How did Drupal 8 end up so different from Drupal 7? It might have something to do with how decisions were made for the project. If we disagree with the outcome of the organic decision making process of the Drupal community, we will adopt a different process when making decisions about the direction of Backdrop.
Read more about our leadership structure.