Because 5* corvus is significantly better then 4* corvus. He is all about killing the other champ in the first 20 hits, and a 4* corvus doesn't have the damage to do that in many cases.
Medusa however continues to be useful the entire fight, and you will be able to use her to do things like 100% the final chapter of act 5. And if you ever outgrow her as an attacker, then you can always put her on defense. Something that cant do done with corvus.
Medusa can armor break just like Corvus, can even armor shatter to prevent future armor ups, when duped gains furies and gains autoblock, AND is poison immune.
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Medusa however continues to be useful the entire fight, and you will be able to use her to do things like 100% the final chapter of act 5. And if you ever outgrow her as an attacker, then you can always put her on defense. Something that cant do done with corvus.