When it continues to fail over and over again despite numerous chances to succeed in some way.
When the creative folks behind the franchise find it unfulfilling and unexciting to keep it going.
When the IP is over 6-10 years old and you've completely wrung the core concept dry and can't do anything more with it besides repeat yourself.
At least those are the most logical and understandable reasons in book anyway.
This is pretty much the only correct answer as there is no point in create new games in the franchise if sales just aren't there to support it.
A franchise ending for creative purpose is very rare and isn't permanent.
Sad but true, especially when it's in the hands of a big cooperate entity.
Money comes first with these guys, and most of the time they don't care about staining the legacy of a beloved franchise for the sake of potential profit.
Like it's not a game series, but the
Powerpuff Girls is probably the perfect example of this phenomenon.
This transgressive acclaimed cartoon show had a nice long 7 year run, and the original creatives behind it had exhausted almost every possible creative avenue that they could take with the core concept. Craig and co. were fine with the show's cancellation, they were done with it and wanted to move on.
But despite all of this, CN brought it back two times...once for a mediocre TV movie that no one remembers, and then again as a sort-of-kind-of continuation/soft reboot that's incredibly boring and derivative.
There was almost no good reason to bring the series back outside of keeping that sweet, sweet merchandising money flowing in.