But why can't user properties be modified from the portal interface?
Because your subscription is set up to synchronise user properties from your user directory (Office 365 or Gsuite)...
In this context, as properties on the Boost My Mail side are updated nightly by "copying" what your corporate user directory contains, we don’t let you modify them from the interface; otherwise they’d be lost!
If you need to change any information, you must request it from your ISD. It will then be available in Boost My Mail the next day.