Anime based on a successful manga series sometimes run into an awkward situation where they have to use “filler episodes” in order to give the manga time to add more story. These episodes are generally more of the same stuff fans already love, but in the back of their mind, they know nothing truly groundbreaking can happen in them.
Gintama is a special case though since the series is quite episodic and those in charge of the anime are free to add their own additional stories. Which is probably why one of the best stories from the original manga hasn’t made it to air yet. Gintama fans don’t have to wait too much longer, though, as the Assassination Arc is finally making its debut on TV this December.