The thing is, Batman will always be easier to cast than Supes because Supes doesn’t wear a mask, and most people tend to want a very specific look for him. So it’s hard enough to find someone who fits that look, and when you throw acting ability and inherent wholesomeness on top of that…when someone checks all those boxes, it’s likely that the fan faves will align with the studio/director faves most of the time because those actors are just rare, and really stand out when they show up, as they make everyone watching them go “that guy is Superman.”
With Batman, most regular-looking actors can be made to look like a pretty effective Batman with the right cowl. That’s why the various cinematic Batmen we’ve had so far have looked almost nothing alike. So there’s a much larger acting pool to draw from that’s narrowed down by the director’s personal taste in actors, which is why the fan-casting is way less likely to align with who’s seriously in contention. There are simply way more options at any given time.