A bit of background for those unfamiliar with the SA. They were the Nazi parties militant wing early on, before the SS took over that role. They had been made an almost unofficial police force by Herman Goring, the SA was responsible for breaking up anti-Nazi rallies or demonstrations by other non-Nazi groups. They attack whoever the Nazis wanted. When Hitler wanted to pass the enabling act, it was the Brown Shirted SA storm troopers who filled the Reichstag and intimidated the opposition.

Now the SA had drawn the anger of two powerful groups, the Army under Werner von Blomberg, and the SS under Himmler and Goring. The Army feared the SA's growing power, and the SA had declared that the Army was simply a training ground for the SA, and Ernst Röhm, the leader of the SA had said that he would soon absorb the Army. Now the SA and the SS rivalry was due to the SA's perceived position as the most powerful Nazi organization.

They also began to draw Hitler's anger. Their over the top rhetoric about a second revolution or finishing the job, angered Hitler who wanted to stabilize the regime and the country after his rise to power. Hitler, Himmler, the SS, and top army officials began to devise a plot under which they manufactured evidence that the SA was planning a coup. Hitler ordered the SA leaders to meet him at a resort hotel, Goring was sent to Berlin to keep order there.

Now rumors started circulating, and SA men did take to the streets in many cities in Germany, notably Munich where 3000 rampaged, shouting that they would fight back if there organization was betrayed. They were eventually calmed by a local leader in Munich. When Hitler heard of this he advanced the coup forward.

One of the main reasons the SA didn't rise up against the Nazis was the sheer speed with which the top SA brass was arrested. Here is what Hitler's driver said happened:

Taking no notice of me, Hitler enters the room where SA-Senior Group Leader Heines is lodging. I hear him shout: ‘Heines, if you are not dressed in five minutes I’ll have you shot on the spot!’ I take a few steps back and a police officer whispers to me that Heines had been in bed with an 18-year-old SA Senior Troop Leader. Eventually Heines comes out of the room with an 18-year-old fair-haired boy mincing in front of him. ‘Into the laundry room with them!’ orders Schreck. Meanwhile, Röhm comes out of his room in a blue suit and with a cigar in the corner of his mouth. Hitler looks at him grimly but says nothing. Two detectives take Röhm to the vestibule of the hotel where he throws himself into an armchair and orders coffee from the barman. I stand in the corridor a little to the side and a detective tells me how Röhm was arrested. Hitler had entered Röhm’s bedroom alone with a whip in his hand. Behind him had stood two detectives holding pistols with the safety catch removed at the ready. He had spat out the words: ‘Röhm, you are under arrest.’ Röhm had looked up sleepily out of the pillows on his bed. ‘Hail, my Leader.’ ‘You are under arrest’, Hitler had bawled for the second time. He had turned on his heel and left the room. Meanwhile, upstairs in the corridor things have become very lively. SA leaders are coming out of their rooms and being arrested. Hitler shouts at each one: ‘Have you had anything to do with Röhm’s machinations?’ Of course, none of them says yet, but that doesn’t help them. Hitler mostly knows the answer himself; now and then he turns to Goebbels or Lutze with question. And then comes his decision: ‘Arrested!’

The SA had not prepared for the speed with which the SS acted. The SA weapon caches were taken, airports sealed off, homes and barracks occupied. The SS also gave justification that made uprisings hard to defend. Rohm had been accused of being a traitor as had other SA men. An uprising in defence of these people would have looked extremely bad. More importantly, with its top brass gone the SA had no one to lead them, the Army and SS were better equipped and better organized, they would have easily dealt with any SA uprising.

The SA wasn't the only one purged. Important Jewish leaders, old conservative politicians, monarchists, anyone unfavourable to the Nazis was purged.

Sources:

Third Reich in Power by Richard Evans

Hitler by Ian Kernshaw