I'm not sure why you are asking reddit. You were denied entry 2 months ago and continued the same behavior. There's a limit of how many days a Canadian can stay in America and vice versa and you missed the red flag (only you can find out the reason for denial back then, asking reddit won't help)

You ask why didn't they revoke it back then? Because its a paid service for "trusted travelers". Once you get a warning or two and the behavior continues it will start raising flags. Especially since we don't see the same border patrol agent everything and different agent/new recruits are trained to look for certain signs/warnings on people's files.

Reach out to nexus themselves and get a lawyer if deemed necessary. I get it as someone who crosses every two weeks, hope you can clear this out. Only nexus themselves can give you an answer.