What isn't to understand? 32 teams with 53-man rosters where the average career length is under 4 years and most players make right at or close to the league minimum. The short career window makes it especially hard for lower tier guys to risk missing games or seasons supporting a labor action that won't benefit them for long. We've all heard NBA players talk about "legacy" and "those who will come after us". The vast majority of NFL players have no time for that.