Because it's better in term of health of the fediverse if we don't end up with only a few big instances like lemmy.ml. Also people can interact with lemmy.kde.social while not being on that instance
So full disclosure, the instance is not 100% official since it is not hosted on kde infrastructure, as this would create too much work on KDE's sysadmin to maintain another service.
But I manage it, and I'm a long-term KDE contributor (5 years now), maintaining most user-facing websites and various applications (Kalendar, Tokodon, NeoChat, Kontrast, ...). I'm part of the KDE Promo team (I have access to all the existing kde social media accounts) and of the fundraising team. And in the past, I was moderating r/kde, but I deleted my account a while ago already. And finally, I already maintain https://kde.social, which hosts the mastodon accounts of a few kde applications as well as of a few other contributors. (all that in my free time)
Hopefully, this is legit enough :) And I will add the link to the Lemmy instance in the footer of kde.org
Because it's better in term of health of the fediverse if we don't end up with only a few big instances like lemmy.ml. Also people can interact with lemmy.kde.social while not being on that instance