They talked about this in a Live Letter or interview from the era following the original EX primals (which were originally in the DF right away). I can't remember their exact reasoning for not putting future EX primals there right away, but it had a lot to do with the demanding nature of the fights, which make them not as suited to the Duty Finder before strategies are developed that more of the community is familiar with.

As for Savage—it's not even on the Duty Finder instance server right now. They have a separate instance server solely for whatever the highest-tier raid is, so that progression groups don't have to content with additional lag and the like caused by heavy server loads during primetime. We can reasonably expect that unless they get an all new instance server infrastructure, the top raid will always be separate from the Duty Finder on release.