Because the current version (if you downloaded now) doesn’t work correctly. They are waiting on the App Store to release the working version they have submitted already. There is a brief turnaround time when submitting apps for download during which the apple team (or google) reviews the app and approves it or says it’s not ready yet.
Thunkable Live doesn’t seem to run well/at all on version 10 of iOS, based on other posts on the forum. (And I can personally confirm it won’t run at all on 9.) Can you upgrade the version on your phone, or are you maxed out?
Thunkable Live runs nicely on my iPhone SE (the 2016 edition, not the fancy new one). In the US, you can get a used but functional SE or 6s for <$100, and those can be updated all the way up to IOS14, so while they’re not super fast, they’re likely to be compatible for a while. Sorry, I don’t think that’s the answer that you wanted.