I originally submitted pyscript as a PR to HASS core, but it was rejected. Here's what @balloob said:

Hey @craigbarratt, thanks for your PR. This is not a feature that we would want to add to the main Home Assistant repository at this point. Using custom integrations for automations or our existing Python Script integration never really got any traction and I doubt that this would go any farther, so wouldn't want to take over the maintenance burden by adding it to core.

Obviously at the time it was a relatively large and complex integration from an unknown author, and @balloob's explanation seemed very reasonable to me. HACS provided a good path to make it available to the community. @balloob was quite supportive in providing feedback and mentioning pyscript to the community after it became available via HACS.

It's unfortunate that the default integration requires a high level of expertise to be useful, and doesn't make it easy to write compact automations. So it's no surprise that HASS hasn't had much adoption of python as a user scripting language, which is exactly what pyscript was designed to overcome.

If there was enough user support, perhaps @balloob could reconsider at some point?

I can't support this enough. Pyscript has been a game changer for me. It is amazingly powerful yet super intuitive, simple and reliable. The fact that I can just write a python script with direct access to the HA internals, put it in the pyscript directory and it runs is fantastic! And the combination of not needing to restart anything to just add new functionality as well as the ability to test and code with Jupyter notebooks is pure joy and actually even more powerful than the already super useful YAML template editor. So thanks so much @craigbarratt for what you have done and of course huge, huge kudos to @balloob for everything you've done with HA and if the two would natively marry, it would just be a dream wedding I am sure for many!