According to the docstring of company-backends:
Only one backend is used at a time. The choice depends on the order of
the items in this list, and on the values they return in response to the
‘prefix’ command (see below). But a backend can also be a “grouped”
one (see below).
So I guess it is because the order.
I said last night that it didn’t work for me because I wasn’t editing a file inside the roam folder. It turns out it wasn’t quite right. I noticed that I have to open a roam file using the C-c n f (org-roam-node-find) function for the completions to work. If I open the very same file, for instance, from the command line (emacs ./org/roam/somefile.org), or using emacs find-file, no roam completions work.
Any idea why?