So I originally also had a look on how we could integrate with existing solutions, but I’ve not found a nice solution that will make it feel like a native Nextcloud app. For wekan in special there is a whole different server stack required for deployment, so setting this up besides Nextcloud is a huge effort for sysadmins. (See Install and Update · wekan/wekan Wiki · GitHub) Yes, for kanboard that would be easier, but you still have to maintain two different software installations.
Another reason for creating a separate app was that other external integrations like web mail clients we had in the past were also lacking a lot in terms of user experience and the integration with Nextcloud. I guess the Nextcloud mail app was developed for a similar reason.
There is no reason for me someone may not start working on integrating one of those solutions to Nextcloud, it was just that I expected it to work nicer if I start with a native app. (And I also mainly started this as a learning project for angular js 
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I hope I could clarify things a bit.
That bug will be fixed in the next bugfix release.