Answer by taking a quote from the following article:

Arnold Jacob Auerbach was one of the four children of Marie and Hyman Auerbach. Hyman was a Russian-Jewish immigrant from Minsk, Russia, and Marie Auerbach, nee Thompson, was American-born. Auerbach Sr. had left Russia when he was 13, and the couple owned a delicatessen store and later went into the dry-cleaning business. Little Arnold spent his whole childhood in Williamsburg, Brooklyn, playing basketball.

Prior to the 1950-51 NBA season, Walter Brown, owner of the Boston Celtics, was desperate to turn around his struggling and financially strapped franchise, which was reeling from a 22-46 record. Brown, in characteristic candor, said to a gathering of local Boston sportswriters, "Boys, I don't know anything about basketball. Who would you recommend I hire as coach?" The group vociferously answered that he get the recently available Auerbach, and Brown complied. In the 1950 NBA draft, Auerbach made some notable moves. First, he famously snubbed Hall-of-Fame New England point guard Bob Cousy in the 1950 NBA draft, infuriating the Boston crowd. He argued that the flashy Cousy lacked the poise necessary to make his team, taunting him as a "local yokel". Second, he drafted African-American Chuck Cooper, the first black player to be drafted by an NBA club. With that, Auerbach effectively broke down the color barrier in professional basketball. In that year, the core of the Celtics consisted of Hall-of-Fame center Ed Macauley, Auerbach's old favorite McKinney, and an unlikely addition, Cousy. Cousy had refused to report to the club that had drafted him (ironically, the Blackhawks, Auerbach's old club), and because his next team (the Chicago Stags) folded, he ended up with the Celtics. With Auerbach's fast-break tactics, the Celtics achieved a 39-30 record but lost in the 1951 NBA Playoffs to the New York Knicks. However, the relationship between Auerbach and Cousy improved when the coach saw that the "Houdini of the Hardwood"--as the spectacular dribbler and flashy passer Cousy was lovingly called--became the first great playmaker of the fledgling NBA. In the following 1951-52 NBA season, Auerbach made a remarkable draft pick of future Hall-of-Fame guard Bill Sharman. With the high-scoring Macauley, elite passer Cousy, and new prodigy Sharman, Auerbach had a core that provided high-octane fast-break basketball. Other notable players who joined the Celtics were forwards Frank Ramsey and Jim Loscutoff. In the next years until 1956, the Celtics would make the playoffs every year, but never won the title. In fact, the Celtics often choked in the playoffs, going a mere 10-17 in the postseason from 1951 through 1956. As Cousy put it: "We would get tired in the end and could not get the ball." As a result, Auerbach sought a defensive big man who could both get easy rebounds, initiate fast breaks, and close out games.

What was believed to be the reason they never won?
In fact, the Celtics often choked in the playoffs,