There already exists an extremely widely used regular _expression_ syntax designed for brevity. We do not need to standardize another one designed for brevity.
When I asked early on, "what are the benefits of the SRE syntax" I got a strong reaction. To me, the advantages of list structure does not outweigh the disadvantages of having to learn yet more cryptic operator names.
I might as well just use PCREs; at least then anything do I retain can be used outside the world of Scheme.