My guess is that having a set -- any set -- released in proximity to the theatrical release of Creed III was the priority here.
They'll eventually have to release the fifth and sixth films which provides an easy opportunity to redo these discs for a larger set. Not that there is any excuse...
I don't think the cause of this timing is about the anniversary as much as tying in with the new movie. The release date for the box set is February 28. Creed III arrives in theaters on March 3 (probably with "sneak preview" shows on March 2.)
That can't be a coincidence.