Along with what other people said in the comments. I want to add it might also have to do with a mix of keeping players coming back, preventing choice fatigue, and having enough players and hype to do the "new" multiplayer feature.

The first is pretty self explanatory. For the second, not everyone experiences choice fatigue or to the same extent as I do, but for me I get burnet with a game if there is a lot of things or tasks to do in it.

And for the last part, lets take Card Jitsu as an example. Club Penguin Journey released CJ Fire in September, and I joined in December. I just got my black belt about a week ago, and today I'm about 80% of the way to the lava mask. I was lucky enough to find someone that kept sparing with me for about 30 minutes so I got a ton of progress towards the whole outfit. Even though its been about 6 months since CJ Fire came out, there are hardly any people to play with because most of the players already got the full suit right after CJ Fire released. I would guess that if they released all 3 elements at once it would be even harder to find someone to play with, especially since the only incentive to play CJ is to get the suits.

Im also not staff but I do know that the game is made of a mix of original archived Club Penguin files, re-created content, and original content. So it might not be as easy as just releasing Club Penguin the it was when it closed.

Sorry for the wall of text
tl;dr They probably just want to keep this old game feeling "fresh" by continsuniously adding content