They've been very quick at implementing CSS changes in Safari, faster than Firefox in some cases, but yeah bigger web APIs like push notifications have taken too long
That seems like a good guess. I'd also guess that there are a lot more internal politics involved when they're implementing OS-integrated features that are easy to abuse. There are a lot of sites that ask for notification permission that definitely don't need to, and that probably saps some of the motivation for implementation at Apple. It's frustrating for developers with completely valid use cases, frustrating for open web standards, but I can understand being overprotective of user experience. It's a difficult balance.
But Web Push requires changes from the whole browser, perhaps minor changes to the OS, and probably a whole server side implementation.
And with all that cross-team effort going on, it's very easy for management to deprioritize it.