It is likely that Adidas is still reeling from previous copyright failings and has realized the bad optics of challenging the Black Lives Matter brand. Having been caught in controversies surrounding Kanye West as well as the #PayYourWorkers campaign, the entire case was a bizarre and poor decision once again by the sports brand.
Trademark law is notoriously difficult to understand, and the Black Lives Matter organization doesn't have the best reputation when it comes to financial matters. Adidas has clearly been associated with the three-stripe branding, however, the Black Lives Matter stripes were of the same continuous size similar to the sports brand. No matter the outcome, you can see how Adidas thought that the filing was indeed worthwhile.