Because that would mean "Ta káva je opravdu dobrá."
"Velmi" translates to "very" whereas "really" translates to "opravdu".
I would accept "really" in this context. I don't see much difference between "Ta káva je velmi dobrá" and "Ta káva je opravdu dobrá". I'm a Czech native speaker and I would say both without thinking twice :)
@z.linky55 use rather "very" to avoid confusion and bear in mind that "opravdu" or colloquial "fakt" (fakt dobrá káva) can be used in some cases.
Because of the Golden rule. We should not accept an alternative if it is closer to another sentence.
Here the really has an exact counterpart opravdu, skutečně so it should not be accepted for velmi.
OK. Then what does "The coffee is good" really mean? Is it that a particular coffee is very good or is it that the coffee in general is very good?