How, after years of AR15 and similar pistols being ok with or without braces, did we end up with the threat of them being changed to SBRs and maybe AOWs?

How, after years of AR15 and similar pistols being ok with or without braces, did we end up with the threat of them being changed to SBRs and maybe AOWs?
Back on November 23rd several U.S. Senators sent a letter to ATF and to the Department of Justice critical of ATF’s new stance on AR-styled pistols and arm braces. While it may be questionable as to how much effect this letter may have on ATF’s current position on AR pistols, the fact that the letter was sent is significant. It would seem that a regulatory agency can’t just give everyone a different answer on a tech question, after all.