I would suspect it is the NBA's ploy to keep the fans of the non-playoff teams reeled in to the NBA.
The NBA lottery is midway through the playoffs and draft, so the non-playoff team fans need to keep an eye on the playoffs, in anticipation of the draft lottery.
If the draft lottery was held a few days after the season, then there is no reason for the non-playoff team fans to watch the playoffs. Now, the NBA has you and everyone anticipating and thinking about them until the draft lottery and then they only have 1 month to the draft. So, the NBA keeps us in suspense and talking about them for the next month.
So long story short, I think they do it for the ratings.