#3 SummerSlam feud with John Cena
It was only after he defeated John Cena that Kevin Owens was considered to be an elite level superstar. It was only after he was in a feud with Cena that Styles was elevated to main event status. So many superstars can say the same. John Cena is the ultimate litmus test for a WWE superstar.
Were Lashley to be booked in a program with Cena, it would just make him seem like a much bigger star than he is right now.