When porting modules from Drupal 7 to Backdrop CMS, it's natural for people to download the Drupal project as usual, and then start to make changes to the code so it will work for Backdrop. When they have got a working Backdrop version of the module, they are often inclined to contribute it back to the Backdrop community -- which is great!
One of the three minor requirements for a ported Drupal project to be included in the Backdrop contrib-group is that the Git history be included from the Drupal project.
The most important reason we want to keep the history is because we want all the Drupal contributors to get credit for all the hard work they have done on the project. Keeping the history can also make it easier to cross-port fixes from the Drupal project, or even maintain both projects at the same time.