Because who knows when or if it will end up in MAME git. I'm not sure what license crt-geom is, or whether JD wants to add every random shader to MAME or not. I think crt-geom would be great to have, personally, because it's got to be the most commonly used of the GLSL shaders and people trying to run HLSL on a potato will have more luck with crt-geom.
There should be no problem with the license:
* This program is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License as published by the Free
* Software Foundation; either version 2 of the License, or (at your option)
* any later version.
and cgwg always gave us kindly the permission to use it.
i agree with the low-resource statements of crt-geom. it is really a good alternative for low-end setups.