First of all, no one wants their anniversary to be on Valentine's Day. It's supposed to be something special and unique to you and your partner. It shouldn't be on a day that's celebrated by literally everyone.
Second, Valentine's Day can be enough to coordinate and plan without a proposal getting thrown in. What if the food isn't good? Or weather delays a flight?
If you are thinking of proposing on Valentine's Day, give it a few days, maybe even a week.