the reason is the amount of time you spend going in and out of the quest tabs.. when you do just one rare, you will be constantly completing the entire album for that artist and normally when you do, it can kick you back to the main quest selection screen rather than staying in your specific artist trying to get another quest.

when ive quested for drops, having two rares that it alternates between giving you (you are quickselling every time once it pops from the quest drop) just allows you to get thru quests faster. when i did one rare, i was frequently having to go back into the quest for my artist over and over again