I think when it comes to adaptations, even the ones that are more serialized like animated series or live action shows, they have to be more pragmatic in what they adapt. The villains are easier to adapt because they can be part of self contained stories and float in and out of the plot as need be. With Batman's inner circle of allies, especially the big amount he has currently in the comics, I can see how it'd be a bit harder to juggle a large cast that needs to have plot relevance to the main character and add something to his narrative while having ones of their own. This isn't as much of a problem with comics where each of them can be spun off into their own comic and get development, but more of a challenge in a film or tv series.

This gets harder the deeper you get into the Bat Family. To adapt some of them properly, you need to have an established Batman who has a wider history behind him, as some like Tim and Cass would be inheriting mantles of other characters who have histories themselves. If an adaptation wants to streamline things down to the bare essentials and have a freer Batman who isn't bogged down by the same events he is in the comics, it makes sense that often the first thing to go is most if not all the extended family, or they get mixed and matched. The DCAU only had Dick, Babs and a Jason/Tim hybrid and they all played essential roles (ymmv on how it all turned out for them of course). If we got more added to that mix, I don't think the family dynamic would work as well. That Batman feels pretty complete. Similarly, Nolan's films don't require a Robin-type role until the end and they only needed one character to fill it. I wasn't personally thrilled with John Blake, but I can why Nolan and co felt they could tell their story with one composite character and get the point across. I hope Reeves uses Dick and Babs at least, but if he doesn't go beyond them (or even avoids using them at all) his vision can still be complete in itself. Because for the bare essentials for a version of Batman, I think all you need is Batman himself, Gotham, Alfred, Gordon and a villain. Even a Robin isn't 100% necessary at this point, though personally I don't think Batman is complete without Dick and Babs at the very least.

The villains meanwhile are freer to show up in any order and easier to adapt.