Because a mall store needs to either sell higher priced items or large quantities of items to stay in business... The average person buys shoes MAYBE once a year, it's obvious they don't move large volumes.
Not only that, but selling insoles in a shoe store is like saying "whatever you buy here... won't last."
And to top it off, the value of items at foot locker is a bit higher, having $5 packages of Dr. Scholls sitting around would devalue the names "Nike" "Adidas" and everything else.