It could be because using degrees per 1000m is appropriate to SI units whereas degrees per 1000 ft is not. I suspect that the old school of imperial measurements left it at 1.5 as it was easy to remember yet the new school JAA etc have complied with SI units but converted it to deg/1000' for ICAO commonality.
Out of interest the SALR curves are usually seen on a tephigram and the SALR rises with altitude as less moisture is avaliable to condense and release latent heat so the parcel of air behaves more like a dry gas.