You won't get an answer from the Discord themselves. But here's my take, as a game developer who was offered to sell on Discord: Discord was trying to turn their massive userbase into a source of revenue with the Discord Store, but they didn't quite anticipate what developers would actually need from a storefront, and they didn't seem interested in understanding the storefront wars of time past, the many "Steam Killers", GoG, Desura, Origin, UPlay, Windows Store, the list goes on.

Basically, no AAA partners really care, they've all been burned before, so they do the old standby when you need to rejuvenate a platform (like the Xbox 360 and PS3), and turn to the indie market, in hopes that would showcase a more cooperative community desperate to get any press or attention they can. But the indie market already has itch.io, and Steam, and Discord didn't seem strictly better than either of those, so it was already a bit underwhelming of a launch.

Around the same time, the Epic Games Store launched, offering much better royalty deals than Steam/Discord, and Epic offered large grants to developers in exchange for limited-term exclusivity. This got a lot of developer/gamer attention, so quickly all the renewed "storefront war" was seen as Steam vs. Epic, with maybe GoG/itch.io as a very distant third.

Discord's Store had no chance out of the gate. They tried to pivot to "buy a Nitro subscription, get some free games", but between their awkward Store interface and nothing really that great on offer, they eventually shuttered it. [0]

[0] https://blog.discordapp.com/whats-coming-for-nitro-a732ddc4b...

Hm I've never heard of itch.io until I see this comment

It wasn't removed secretly; they refunded everyone, wrote a blog post or two about it, and floated the news to the top of most news aggregators at least a few times.

I got that email and cancelled my nitro subscription because of it, but I didn't know the actual store shut down until months later.