Reason #2: Timbo and Missy both worked with Aaliyah extensively and understand what she would have wanted to do with her unreleased music.
Outside of just being familiar with Aaliyah's unreleased music, Timbo and Missy also know and understand her sound like no one else. Part of that is because they both helped shape her sound. But, part of that is also because they spent so much time around her. So, they'd know best what she was trying to do on certain songs and how she wanted things to turn out on individual tracks. It'd be difficult for them to complete those recordings. But, at the very least, they'd have a blueprint for how to do it in their minds based off what they know about Aaliyah.