As @Peter Shor mentions, the pattern that is present in all languages seem to prefer the positive over comparative and superlative. Trying to explain why this is could be intereseting because it seems so evident. On my part I can only offer speculation:
Wiktionary list "good evening" as ellipsis of a phrase such as
I wish you a good evening.
This seems obviously true and nothing new. Now, using comparative would be confusing here since "I wish you a better evening" would imply that it might have not been the best evening so far. Actually, we tend to use comparative in situations when we want to console someone e.g. "I wish that things will be better."
Superlative would work, there is nothing confusing in the wish
I wish you the best evening.
and it is a very nice thing to say when you actually have a reson to do so. However, using this phrase everytime would eventually be insincere as you need a way to say goodbye even on the nights that are not so great or you might want to be civil with your enemies, too.
So, the choice of good seems really the only way, and really what is wrong with wishing that something would be good? I think it hits the spot perfectly, leaving the possibility that it might turn into better and best.
If you go by etymology, then
also goodbye, good bye, good-by, 1590s, from godbwye (1570s), itself a contraction of God be with ye (late 14c.), influenced by good day, good evening, etc.
might seem like another possible explanation of "good", but the mention that good bye is influenced by good day and good evening immplies that these were already established, which the entry for good day confirms
salutation, late 14c., short for have a good day (c.1200). Good morning is c.1400, gode morwene. Good night, also goodnight, is late 14c.; as an exclamation of surprise, from 1893.