He probably don't have any ideas of next aliens missions or good development outside missions, so he keeps kannon for a long time untill he gets a better ideas or the manga is cancelled.
In the original, the Kannon couldn't regenerate after the clock-like object in one of his hands was destroyed. In this universe Kannon seems 100x powerful for some reason.
In the original, it was the Gantz person inside the ball that decided when the mission finished if not counting the time, and that person always comitted some errors, like transporting Kishimoto when she was actually alive from the suicide attempt.
This manga will have this mission and probably one more and will be canceled, like Gantz:G.
I hope that I'm wrong. Buy the original rich guys.
~ Thanks for the Chapter ~