Harry Reid, while a rabid mouthpiece for the Dems, is from Nevada which has gone from safe-Democrat (southern-style) to safe-Republican over the last ten years. He has to protect his seat, and since Nevada is as pro-gun a culture as any State in the Union, he has always voted against anti-gun measures despite being one of the most senior figures in the Democrat Senate. Being anti-gun is a surefire death sentence for a Nevada politician no matter which party line you toe.
Reid's powerful hold on his seat is already starting to get shaky due to a demographic shift. Voting for an anti-gun measure would have allowed a Republican contender to bludgeon him mercilessly with a Nevada hot-button issue. Smart politicians know how to survive.