Johnson, on the ballot in all 50 states and Washington DC belongs, Stein, on the ballot in 36 states, does not. Having a commission created by Republican and Democrats decide what the criteria is for being part of said debate solidifies the two party system.
1. Be eligible to be President
2. Be on the ballot in enough states to be able to obtain the needed 270 electoral votes to win
3. Be polling 15% of higher in an average of five national network polls
Johnson and Stein both met the first two, failed at the last one. It's not a new requirement added this year, it's been 15% for a while. Johnson and Stein knew what was required of them to get in.