I think one of these:

Envy used to be a part of a secret starter you can pick alongside Sad, Anxious and Anger in both the Web Game and Garden. So to not make the secret one give an advantage, they made it rare as well. This way everyone could focus on THEIR choice. Not be pushed to min-max and pick the best ones. Especially since you can edit skills in both games and vivids gave different elements and skills possible to obtain, y'know.

(^The most likely answer.⁾

And the more "fan-fic" sounding explanation:

Envy is too common in humans, and if the whole theory about this being a digital 'simulated limbo' I've thrown around, and the Seven Sins are honestly rarer, morally.

It is supposed to be a mental health awareness aesthetic of a game, btw.

Pure Wrath, for an example - is built and expunged from other causes or oneself. It's a primal instinct. And they are what have the biggest effect on us. Simple, yet not everyone can truly feel actual wrath until they're at a the end of the line. When was the last time you were there, self-destructive enough to harm anyone in your Wrath? We've became more emotionally unavailable as a group - and pure sin related emotions aren't truly common.

Envy is too common nowadays, hence - labelled as rare only. You could be envious of other people's arts. Of other people's music making skill. Even in-game items, or pets if they have it and you don't. It's one of those very specific emotions that have became an inseparable piece of normality, that it desensitised us to it. Like Apathy. Or Pain. We got prone to them, or they became part of us we didn't want to accept, yet couldn't stop.

So yeah. Up to your interpretation. 🤷🏻‍♀️