That's not true: IntelliJ was the first one to come out licensed as Apache 2, and PyCharm (and the python plugin) followed suit. Both of them do have paid versions with more features, but IntelliJ and PyCharm open source builds are still very strong products and unquestionably not "phoning it in" releases
PyCharm lives in the IntelliJ CE repository, both are Apache 2.0 licensed, while the rest of JetBrain's language plugins are proprietary. They are 'unifying' PyCharm and getting rid of the separate PE and CE builds, but the underlying plugins remain developed in the open right there on GitHub.
As far as why only IntelliJ and PyCharm are done this way (excepting additional plugins many developers would end up wanting only coming with a subscription), that's a strategy question you'd have to ask an exec about. If I would hazard a guess, Java, Kotlin, and Python are all pretty big first languages to pick up and have large, active communities. Good products to get people hooked and want to pay to get the extras that aren't available without a subscription.