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 bares very little resemblance to the Drupal of yesteryear. 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 we forked: to be able to update and improve as necessary.
2) Decision Making Process
How did Drupal 8 end up so drastically different from Drupal 7? It probably has something to do with how decisions are made for the project. If we disagree with the outcome of that process, perhaps we need to take a good hard look at how that process works, as well. See our leadership structure.
3) Diverging principles
The issues that are most important to the Backdrop community may not be as important to the Drupal community, and vice versa. When the principles diverge, so will the code. See our principles for a detailed look at what Backdrop stands for.
4) Target Audience
Backdrop CMS is specifically targeted at smaller businesses, non-profits, and education. Backdrop best serves the kinds of organizations that need complex functionality, on a budget. As changes and improvements go into Backdrop CMS, it's these people we'll be trying to help the most.