If you subscribe to the TestFlight beta, the new build does offer support for the MainUI.
As for why it’s taken so long, the simple answer is because openHAB is open source, and the developers are not paid. Therefore, they work on whatever they want to, and us users get to benefit from using awesome software for free. I get that it’s frustrating - I’ve certainly been excited to use MainUI on my phone and have actually been running everything in sitemaps even though I switched to OH3 when it was released. However, I don’t have the skills to develop, so I’m just resigned to be a grateful user (and try to help on the forum when I actually have time).
I believe the iOS app had several developers get too busy with their personal and/or work lives to keep contributing, and it wasn’t actively progressing for several months this year. Fortunately, it has been making some forward progress lately, and by joining the beta program you can help identify bugs to help the developers deliver a better release. Personally, I’ve not had any issues with using the MainUI in the beta version of the app and use it daily on my primary phone.
If you want more behind the scenes updates, subscribe to updates on the Github repo.