To me, it was clear... and many have said it... failure to bring in an experienced NFL OC with a proven track record of success.
Just looking back at his OCs tells you everything you need to know as to why he is no longer employed by the Rams.
It really is this simple, although I might tweak it to say that it didn't necessarily have to be an experienced OC, just one in tune with modern NFL offenses (note that Martz had zero experience as an OC before he was hired by DV). But yes, his OC hires went from bad, to worse, to gawd-awful, and it was clear that between his ultra-conservative offensive philosophy and his unwillingness or inability to land a good OC and then just leave him alone to do their thing instead of trying to imprint his philosophy on them.
As far as McDaniels goes, I can't see him working well with Fisher anyway, since Fisher's player acquisition philosophy is to emphasize athleticism above all else while McDaniels offense emphasizes getting smart players. That's why they get great production out of smart guys like Chris Hogan. Under Fisher the Rams chose athletes like Brian Quick who never gave us any consistent production.