Seems to me like it was just abandoned by the dev. On GitLab, the repository is archived. KDE already has several other chat clients which use more open protocols, KDE devs are probably more interested in using and maintaining those. Most of KDE's official channels are on Matrix, which's why NeoChat gets the most attention.
In addition to the previous comments, the official telegram-desktop app is also Qt based and open source. So maybe the dev thought it to be a lot of duplicate efforts.
Only one that's not mobile optimized is the telegram desktop client. All the mobile clients are. But yes, if you are looking for an app specifically for Linux Mobile use, you are out of luck, currently, although telegram-desktop functions alright on a small screen.