Unlike Primarch Rogal Dorn of Imperial Fists, Dark Angel Primarch Lion el'Jonson never had his ass kicked by Primarch Night Haunter of Night Lords (do we see a trend here?).
When the Emperor had to have one of his Legions stay behind and guard Earth while the others traveled the Galaxy gaining fame and glory, Rogal Dorn jumped up and down shouting "Pick Me, Pick Me?I'll do it?"
Dark Angels have the greatest victory record of all the Emperor's Legions. Imperial Fists are 0-1-1 (no recorded victories, lost to Iron Warriors and they like to call the defense of Earth a "Draw".
Lion el'Jonson fought Leman Russ toe-to-toe for a day with neither Primarch besting the other. Rogal Dorn would have used the famous "Play Dead and Run Away" tactic.
Dark Angels would have NEVER screwed up the defense of Earth and got everyone killed. And where exactly was Rogal Dorn when everyone else attacked Horus?.