I think the only time the show has ever mentioned that a game has been "retired" (other than when a previously retired game is brought back, like Check Game) was in the 1990s episode of Decades Week - Drew mentioned that the five starting symbols for Cover Up were five of the six games retired in the 1990s (Gallery Game, Split Decision, Bump, Super Saver, Super Ball - the sixth was Give or Keep).
Here's a similar question: just when is it decided that a game should be retired? When did somebody suggest, "You know, we don't really need Hit Me any more"?
I can only offer speculation to your question.
I imagine several factors are considered when determining whether or not a pricing game should be retired.
Win/loss ratio for example. If a game is lost too often (or perhaps even won too often), that may factor into retiring it. (Bullseye I/Double Bullseye)
Time constraints, especially since practically 30 minutes of the 60 now are commercials or bumpers. Credit Card, a 5 prize game is gone due to this.
If the game is deemed too difficult for players to understand. Mystery Price is a good example I'd say. Hit Me is another.
Inflationary reasons may be considered, such as Poker Game. In 1975 playing for four three-digit prizes was big, by 2007 it wasn't anymore.
Audience feedback, although maybe not as much now, but Shower Game was axed early on due to negative audience feedback.
The production staff dislikes a game, which is why Give or Keep was retired, as well as Telephone Game.
The game may not really fit the format of the show, Professor Price using trivia questions is a prime example of this.
Some pricing games led to only partial wins, Step Up was a prime example of this. I know Roger said he preferred games that were win or loss only.
Prop malfunctions and/or destruction sometimes factor into games being retired (e.g. Penny Ante, Hurdles).
Some times the host/executive producer (Mr. Barker) would strongly recommend getting rid of a game due to him disliking it (Finish Line).
Some games are almost duplicates of other games (Clearance Sale was Easy as 1-2-3 but with price tags instead of blocks).
The host making an error with rules of a game (Barker - Super Saver, Carey - Make Your Mark).
Although I did do a post recently stating that I wish it were possible to have even more pricing games than the 77 currently used, certain pricing games featured on the show could be retired and I wouldn't mind at all. That's Too Much! for one, Stack The Deck (unless they revise the rules somehow), Coming Or Going, Pay the Rent and One Wrong Price are all games that could be retired and not missed at all by me.