One reason could be to not let spam reports on dropboxmail.com affect the primary domain's reputation (which is also used for corporate communication). If for any reason, the reputation of dropboxmail.com is compromised, they can move on to dropboxmail-1.com etc.
I began investigating DNS records to determine if there was some way there to conclusively ascertain that dropboxmail.com was legitimately owned by Dropbox. I could not find a way to determine this through DNS.
Next, I went to dropboxmail.com in my web browser, which redirected me to a page on dropbox.com [0] assuring me that dropboxmail.com was owned by dropbox.com.