Why can't I configure Public Transport / Transit as my default travel option?
Allowing setting Public Transport / Transit as the default for events is something that we'd certainly like to be able to do. The reason we haven't done it yet is because Apple's coverage of public transport travel information is so limited geographically. If you set it as the default, and then moved to an area that doesn't have coverage (sometimes even just a few km/miles away), then we'd not be able to automatically get those transport details for you and set the alarms appropriately.
Having said that, it is something we'd like to improve. We have an entry on our Feature Request board for this very thing: https://bonobo.canny.io/timepage/p/set-public-transport--transit-as-default-mode-of-transport. Please add your vote here to ensure you know about any developments.