Challenge as a whole prides itself on being an archive of popular game shows. But there are so many of them that it's important they strike a balance between "showing new content that may be popular" and "rehashing the same old content over and over again". While I'm sure many of us would love to just watch Price is Right, Play your Cards Right, Knightmare, Millionaire and Bullseye for weeks on end, I'm sure many would like to see a refresh of content every so often to keep the channel fresh for others.
I would really like to see some game shows on Challenge though. Things like Winning Lines and Test the Nation were pretty good shows, but I can appreciate that for Winning Lines they would have to show a very heavily edited version which doesn't include the Lottery draws, and Test the Nation probably is lost to history.
How do they decide on what game show to chose, and where do they go to get the rights to air them?
Like all commercial operators the decision is made primarily to attract the most number of viewers in any particular hour for the purposes of commercial viewership. The more people watching, the more money Challenge get, simple as that. Programmes are only effectively bookends to the adverts to an extent, you sitting there watching The Chase doesn't in itself generate Challenge any money, that comes when the ads actually air and when the viewing figures come in later.
As to where they go, most of the time its the original producers of the show. For programmes that were made by the BBC, they go to the BBC. For ITV stuff made by the former ITV companies (like Bullseye for example which was made by Central) that's in the hands of ITV Studios as a rule (with exceptions) and the independent material (like Millionaire for example which was made by Celador) you usually go to them.
Programmes that were originally made for Channel 4 (and to a later extent Channel 5) follow much the same pattern; because these TV networks were deliberately set up to be publisher broadcasters (ie they don't make anything themselves but commission other people to make things) the rights can vary, sometimes Channel 4 owns the distribution rights, other times the original production company. (likewise independent productions on ITV and the BBC sometimes fall foul of this). In the case of Crystal Maze which was made by Chatsworth TV, that company closed in 2006 when Malcolm Heyworth retired and the programme rights moved elsewhere. It's tooted that Challenge have spent more on the Maze since 1998 than Channel 4 ever did.