Because it is raaacist, just like id for voting.Most states display the full legal name on their driver's license. New York has for years let people carry drivers licenses that in some cases have only a first and middle initial.
I don't know why so many of you are objecting to bringing in a combination of documents to meet the requirements.
The FFL is promoted during the nics check to verify that the ID exactly matches the buyers full name and address. It sounds like some small dealers are ignoring that if they're still accepting ID with initials only