Contamination is a big issue. Lowly McDonalds changed to a 'flavor robot' shake machine, where the ice cream(sic) is always vanilla and they deliver other flavors (chocolate, strawberry, mint etc) through tubes for each dispense.

I like vanilla, but can't get it any more. I get a mixture of vanilla and whatever-the-last-guy-got. Because, tubes leak/drip/have to void of the previous flavor.