I did a very perfunctory scan of sack leaders and what stood out to me is that the AFC is absolutely stacked with defensive ends at the top of the sack rankings. I was only focusing on defensive ends since that's Gary's position.
When I separated out just the NFC sack leaders there were only 4 other DEs that ranked higher than Gary. One was Nick Bosa who made the Pro Bowl) and one was Aiden Hutchenson but everyone knows the story of his injury.
I think it just came down to lack of stand out players in the NFC and whatever impression coaches and players around the league who submitted votes have of Gary.
People can argue other peripheral stats and their meanings but let's be real, these guys are most likely looking at sacks and even if Gary is a couple sacks behind a couple other guys he's still close enough where that might be overlooked against whatever his reputation within the league is.