The official statement goes like this:
This core, not requiring a BIOS, shifts memory too much compared to all other supported cores and would consistently prove to bring issues.
(Scource: www.retroachievements.org)
If it is true and this core generates issues while the others don´t (personally i am using Duck- & Swanstation since a long time, so i dont have own experiences) it seems to be a logical descision.
But of course it is quite annoying for all die-hard-fans of the PSXreARMed-core, as far as they also want to get achievements!