The other thing is that some shows have had multiple seasons in a row in which that show really was the best in it's category. The Sopranos, Mad Men, Seinfeld, Fraizer, The West Wing, all come to mind. True enough the Emmys sometimes misses the expiration date by a season, but I no longer begrudge the multiple winners only because they are multiple winners.
That certainly applies to Dexter. Dexter was nominated for Best Drama in season five for some reason. Clearly they didn't get the memo that season four was the last good season!
Also once a show stops getting nominated it is extremely hard for it to come back as a nominations contender.