Worth to read: Backdrop CMS: The best Drupal alternative for your website.
And here is an information from official Backdrop CMS site.
Upgrade from Drupal 7.
Backdrop has a built-in upgrade path from Drupal 7. Backdrop will be familiar to people with Drupal experience because it is so similar to Drupal, but includes numerous usability improvements and new features.
Serve pages fast, even on shared hosting.
Backdrop is lean and mean. With a commitment to performance, you can expect to see continued improvements in each version. Watch your pages fly!
Add more functionality, quickly.
Browse and install add-ons directly from your own website. No need to scour the internet for available modules, themes, and layouts.
Works great on phones & tablets.
Every Backdrop site is always responsive. Visitors enjoy a great mobile experience, and administrators can create and edit content from phones or tablets.
Create and manage your own content.
Easily create content by formatting text and uploading images. It's not necessary to know web programming languages (or hire someone who does) to get your content on the web.
Control access with permissions.
Different people can have different levels of access to your content. Use Backdrop's robust permission system to define roles that exactly match your needs.
Customize everything, everywhere.
Need to add your own special touches? Backdrop has a robust API and can be modified in any number of ways with only a few lines of custom code.
Security is a priority
Backdrop CMS was designed with robust security in mind. New security issues are addressed quickly and responsibly.
Deploy changes easily.
Make configuration changes on a development site. Deploy those changes to production with the easy-to-use Configuration Management interface.
Backdrop CMS is free software.
Backdrop CMS is free and Open Source. There are no acquisition fees or licensing costs.