Its all about brand recognition.
Halo has Master Chief who is easily recognised just by the shape of the helmet and the visor alone, Halo is also very colourful unlike Resistance 1+2 then we have the theme tune which is catchy and can be recognised by only a few notes plus a great orchestral and at times harmonic soundtrack, its surprising how much sound plays a part in what we like doing.

And of course the game play of Halo is good and whats more, Fun, I cut my teeth on it in PC land.

The box art of Halo games has Master Chief on them

The box art of Sony's AAA shooters (Resistance and Killzone) has the bad guys on them.