I doubt there is any actual fundamental barrier, rather the reason is simply that the primary workload that Linus himself runs benefits greatly from faster metadata lookups, and therefore every nook and cranny of it has been optimized to as fast as possible.

In contrast, while metadata performance is sluggish in NTFS, at least a few years ago there were ways to get better large transaction async file performance than ext4 if you knew what you were doing -- clearly that workload was something that customers had asked for.