Just basic negotiation. The vast majority of players that aren't superstars actually do want 18 games because it's an extra game check, increases the salary cap will likely add additional roster spots which increases the likelihood they'll extend their already short careers.
The 16 to 17 game switch was made almost effortlessly by the NFL. The NFLPA negotiated the ability to not test/penalize marijuana use in return which is a joke. It'll be a very similar situation this time. What they'll push back harder on is the amount of international games, extra bye week and required rest time in between games.