People usually prefer static party or recruit members from among friends, because it is a big headache to do difficult content with random strangers. Like, for DF, you can't control whom get placed in your party. You don't know if someone will join and make you get no chest for the week. You don't know if someone will join as "duty completion" despite knowing no mechanics. You don't know if someone will be very bad at the fight and keep dying and be a burden.... etc. So, yes, it is natural that you can't get DF or PF.

In PF maybe you can control (a little) whom you get. But it's still a gamble.