It would be because it doesn’t fit Joplin’s model. Notes are just notes. To-dos are notes but with additional properties: is_todo, todo_completed, todo_due, etc. (it’s based on the iCal spec). The todo_due is a date that tells when the task should be done, and that’s what is used to set up the alarm, and notes don’t have this.
I can see why it would be useful but I can’t think of any simple solution (I guess a way that would work now would be to move the task to a notebook that’s not often used, so you won’t see the note)