It's been a while since I've been here, and now it seems that nobody uses CJam or Pyth anymore. New languages have sprung up since then. Has that happened just naturally? Was there a rule change or something?
now it seems that nobody uses CJam or Pyth anymore.
Reports of their demise are greatly exaggerated. The Top Language Golfers query shows that 20 people have posted 86 CJam answers this year; Pyth is harder because the query mixes in Python answers, but with a modification to the script I think it's 34 users and 232 answers. (All numbers approximate, depending as they do on heuristic string matching).
However, it is fair to say that they are in decline: the query shows about 164 CJam answers in the second half of 2017, and about 585 Pyth answers over the same timeframe.
As to the actual reasons, though, we can only speculate. Engagement levels is probably a factor. My own decline from 23 CJam answers to 8 is largely down to lack of interesting questions, as a result of which I'm spending more time on the Code Review stack and less on PPCG.
This is a natural process; newer golfing languages learn from the shortcomings of their predecessors.
When I joined the site, the most popular golfing language was GolfScript. Once CJam, which was heavily inspired by GolfScript, was published, GolfScript got used less and less. This isn't (only) because GolfScript would almost always lose to CJam, but also because it was easier to use because of its built-ins and stricter typing.
Pyth is newer than CJam, but so vastly different from it that many CJam users never switched. It is still fairly common compared to CJam (~250 Pyth answers in 2018, ~100 CJam answers), possibly because of its uniqueness. CJam, on the other hand, is a stack-based language; there are many newer stack-based languages you could use instead.
Although both Pyth and CJam are still both used, this usage has indeed decreased a lot. Let's analyse what happened in Pyth's case, as I don't have much of a CJam background. As a prefix-based language with powerful control-flow structures, Pyth has always been quite unique. Some users that were already actively golfing in other languages existent at the time never actually switched to this language, mainly because it was so different from those that were already existent. There are a couple of reasons that lead to a decrease in activity when it comes to Pyth:
It is ASCII-only, and newer golf-langs such as the aforementioned ones took advantage of custom encodings to vastly increase the number of available commands (this also applies to CJam). This is something that definitely disadvantages Pyth a lot – It lacks some built-in functions that were introduced to those languages later on (notable examples include divisors, exponents of prime factorization etc.)
Some of the most proficient Pyth users have gone quite inactive: isaacg (its creator), Jakube, FryAmTheEggman and there are certainly many more I am omitting. Talking about creators, CJam is in the exact same situation with aditsu.
The last point on this list, and perhaps the most important: they both aren't as competitive as they used to be, often being beaten consistently by other esolangs. I think this doesn't need further explanation.
And while they are less used than they were before, don't be hesitant about having fun golfing with them :). At the very least, I still am enjoying myself when using Pyth.