Because the Sandie+Smiths songs were made available over and over again on other releases, including in alternate versions (remix, radio sessions, etc).
The Nancy-Morrissey collaboration is just one song, and it is only available on the single and LP which are already mentioned on "Passions Just Like Mine". Creating a new section just for that would mean energy wasted for nothing new.
I'm currently working on new sections that will be much more interesting than that. They should be up within a few weeks.
Stephane