I bought a Glacial Bay vanity in dark cherry but the medicine cabinets do not come in dark cherry, why?

That is a question for the store where you bought the vanity.
It isn't uncommon for colors to be dropped/changed frequently. Even a color such as 'dark cherry' can change from batch to batch, month to month. It just isn't wise to try to match colors that you don't buy together.

Ask Home Depot.