Why do you suppose Subway won't put the veggies and mayo on the side?

I always get a 6” sliced turkey, provolone (extra cheese of course) lettuce, tomato and mayo. When I get home I cut it in half and have the rest for my meal the next day.
Well the veggies and mayo make it kind of soggy and nasty the next day so I once asked if they’d put it on the side so I could broil the other half of the sandwich the next day and add the toppings after that.
They said on the side was not allowed. I can’t figure out why.
What do you think?