The first element of leadership in The Walking Dead world is the ability to keep people alive. Group survival is essential.
Rick has done a decent job of keeping the original survivors alive. At least he did until the group attacked Negan and the Saviors. But the people of Alexandria haven’t all survived since Rick arrived. As Spencer pointed out before Negan gutted him people at Alexandria started dying when Rick and the group showed up. Deanna, Nicholas, Reg, Sam and his family, and many others have died.
Rick is not totally responsible for that, since in some cases it was their own mistakes that got them killed. But if Rick is going to claim leadership of Alexandria those deaths are on him.
Negan, on the other hand, has huge numbers of people and has created a very secure community at Sanctuary. The large amount of people they have means that if some die the group is not threatened. There are enough people remaining that the loss of a few won’t have a huge impact on the overall community.
Negan’s Savior footsoldiers do a good job of keeping walkers away from the main Sanctuary. And Sanctuary is relatively hidden from other survivors. The sheer number of soldiers that Negan has combined with the weaponry they have means that no group or walkers are really a threat to survival at Sanctuary. Being proactive on security gives the edge on group survival to Negan.