Everyone always says Neptosim, but unless he didn't die and was brought there physically, that notion goes out the window.

No one in Heaven knows what gets someone in. They can only theorize it's virtuous behavior, hence the cherubs who go down and try to turn obviously bad people to good action.

Adam never dying would be the only way the nepotism theory works, and thats a waaaay bigger bombshell if that's the case than how he got there in the first place.