Basically, that's the reason. The draft age was lowered because of the threat of legal action. Because employment as a player in the NHL is governed by a collective agreement, the league and the players' union can decide whatever minimum criteria they wish for entrance into the league.
Ken Linseman's suit to join the WHA Bulls was part of the legal precedent as a right to work suit. That helped pave the way for the shift.
As much as fans would like to see more surety in the draft process akin to NCAA finished products going into the NFL, it isn't going to happen.