TLDR: "From the players’ union perspective, lowering the draft eligibility age to 18 would bring more young players in to develop in the league and take away roster spots from veterans (and the union is made up of those veterans, not undrafted players). The union has suggested ways to keep veterans on the roster (possibly a roster expansion) as mentors, but a deal could not be reached. As for the teams, plenty of GMs would prefer an extra year to evaluate players, especially with them going up against better competition in college/G-League/Overtime Elite/overseas."
TLDR: "From the players’ union perspective, lowering the draft eligibility age to 18 would bring more young players in to develop in the league and take away roster spots from veterans (and the union is made up of those veterans, not undrafted players). The union has suggested ways to keep veterans on the roster (possibly a roster expansion) as mentors, but a deal could not be reached. As for the teams, plenty of GMs would prefer an extra year to evaluate players, especially with them going up against better competition in college/G-League/Overtime Elite/overseas."