Original sentence: As a result, sometimes a new version's code are not compatible with a previous version.Backward Compatibility (for software development): Drupal's designers have decided that, in terms of programming, backward compatibility may be sacrificed with each major revision.Learning curve: According to blog site newassignment.The policy is for Drupal code developers only, not end users.This means that contrib module and theme developers may be required to re-work some or all of their previous code to ensure compatibility with the latest version of Drupal.net, some users have described Drupal as having a "fairly steep learning curve
Edited sentence: Backward Compatibility (for software developers): Drupal is constantly changing, and it is a tenet of the design team that, in terms of Drupal's programming, backward compatibility be sacrificed if it hinders new features or the aesthetics of the code.Learning curve: Drupal may be powerful, but it is also complex.Many casual users may prefer ease-of-use to powerful features, making Drupal less popular than more user-friendly CMSs.Sometimes, version changes to code are not compatible with a previous version.This means that Drupal developers may be required to re-work some or all of their previous code Drupal's policy regarding backward compatibility is for Drupal code developers, not end users