There's no need to enforce it. People who care about lore will seek it out; people who don't care will skip the cutscenes in the sidequest anyway, and it would slow down the pace of the main story – especially for those who come later or are replaying on alts.

I also don't see that much value in knowing the archfiends' stories immediately. They are already dead and the little bit of insight into their past isn't anything we need to know for the next step of the main story. It explains the motivations of characters we faced in the past, not the future.

I also just don't find most of them particularly engaging. With the exception of Rubicante, it was just an extra round of "these are not particularly nice people", and for Rubicante an extra round of "he was good and noble, but you knew that already".