Why do we think that Parcells, "... keeps getting rejected..."? This was his first year of eligibility (again). He gave up his original eligibility by unretiring to take Jerrah's pile of dough to coach the Cows. When he left the Cows, the five year clock restarted.
I think he deserves to be in the HOF, and I'm sure he will be soon enough. However, there can be no doubt that Tuna is viewed by some of the voters as a self-aggrandizing mercenary not a team guy.
A possible answer to the corollory question why was he not a first ballot HOFer is the fact that some voters oppose electing a non-player over a deserving player for one of the very limited number of spot. And this year, there were two non-players: Parcells and DeBortolo who offset each other in that regard.
Cheers!