Tradition, of course.
College basketball had two halves. Currently they are 20 minutes.
When the NBA was formed, it was felt that the two halves would lead to too short an event (remember, college basketball had two games each time: JV and varsity). People didn’t want to pay for just 40 minutes of action (actually, the halves might even have been shorter back then). So the NBA decided that four 12-minute quarters would give a long enough game for the fans to be satisfied. It’s been that way ever since.