The reason is often that writing forces you to already process and abstract the information. Especially if you are taking notes real-time like in a lecture. You will naturally want to shorten the info to write less so you have to process and understand what is the important info, you have to take the info in context of previous knowledge etc. Typing is often much more mechanical, you just need to process the info as it is coming in and transform it into mechanical keypress.
I also remember something about handwriting processing being a nuanced and very separate process from typing, although I am not certain on this. There was also some stuff about reading your handwritten notes triggering memories better than typed notes.