So drill is with the M16 now. (Shrug) Drill and the manual of arms started out hundreds of years ago using spears and pikes. I suppose when candidates/cadets/recruits started drilling with the M1 then there were some mixed emotions about whatever rifle used to be used before that.
Drill will always be important in that environment for the same old reasons- call it teamwork, unit cohesion, pride, discipline... plus it's just such a good way to move large groups of people from one place to the next.