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.