NBA basketball is all about the quest for a championship. There are plenty of small victories along the way, but in the end if you don't win the title most fans are left bitter. Settling for a conference championship just means that you got as close as you could to getting that ring, but failed. Adding to the "win it all or who cares" sentiment the NBA rewards losing by giving those teams better draft picks to potentially add the piece to get you closer to a title.
For that reason, the vast majority of teams with little to no real chance of winning it all in a given season would rather tank than scratch their way into the play-in tournament.