“I can’t figure out why Gary Johnson is kept out if he’s allowed in.”
Same reason Thad McCotter’s left out (and I really like him a lot - he’s a brilliant, articulate conservative, but no name recognition) - each candidate had to have polled at least 5% at some point in order to be invited to this debate.
PMSNBC lowered the bar to 4% for this debate just so they could get Huntsman.