If you expand “change” in the schema table Events and read the descriptions for these fields, they’re pretty conditional. It looks like new_value for changed events only shows if the new value is an Asana Resource. This means it only shows for fields like parent or assignee.
For non-resource fields like description, you currently still need to request the object after the change event to get the new description. It looks like the reasoning for not exposing primitive fields was security related.