Simple, burning hands requires that you be within 15' of the target (if not closer). That is a very unpleasant place to be if you are a squishy wizard/sorcerer.
So, you can either do 5d4 (average 12.5 save for half is 6) to those three enemies and risk getting killed or you can do 10.5 to one of them and be safe.
Typically, I opt for the safe route unless I have specifically built my wizard/sorcerer to be "up front".
Then again, typically I would save a MM for something more important, like an enemy spellcaster.
Pretty much this. The more enemies are within the 15 ft. cone from you, the more reason there is to be anywhere but in range for burning hands, unless you have a very specific build of wizard that pushes the damage of burning hands in big, big ways.