athenes is jsut an older item than morello so its possible that some guides simply havent been updated. But i can see reasons for either.
Lux's thing and what makes her unique among mages is that she basically spams a lot of stuff at very long range...much of it doesnt hit. but she can get people damaged enough before they engage to make engaging a mistake...its basically an extended poking process for lux for the whole game.
in order to perform this poke war, its obviously necessary for her to have max cdr, and its obviously necessary for her to have huge mana regen. Athenes has more mana regen than morellos. significantly more, because of the chalice effect (mana font).
Tweaking the damage a small amount doesnt really matter. what does matter is the spamming where you get people damaged a bit. much of your spam will simply whiff since its long range. its unlikely you kill anyone this way, but you do hope to force them away.
But anyway since the mana regen of morellos isnt bad, i could totally see why you would prefer getting that. But the damage still wont be enough that you are going to kill anyone, and the poke war cant last quite as long becuase you will go oom.
Personally, id usually prefer athenes. Its mroe likely you need to poke for an extended period, and the 2% mana regen every 5 seconds is necessary to support extended poking. If you are never going oom with moros, you probably dont spam with lux as much as you should :P But the greivous wounds effect makes some sense also...my only real qualm is that probably something like 80% of the time the damage you are doing is not on people with low health. and the of the remaining % the people are probably just going to go recall, not get killed. morellos would make you a bit more effective in teamfights, but as a poke champ direct teamfight engages are not really where lux is at her element.
Recently i use lux as support much mroe than a midlaner. In that case i do prefer moros...even though its a late buy...because you dont spam as much. you do a lot of wandering around and warding, and you dont spam q as much because often you need it on reserve in case of emergency.
and 
. But against AP I recommend starting 
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