Dear ICT SLB,
The radio altimeter may be OK for the lower decision heights of Category II operations, but terrain profiles are not always suitable for the distance before the threshold applicable for Category I. It is an option in PANS-OPS for ILS/MLS and GBAS, but I do not know of any State which has provided it. Use for Cat I would require flight checks by the requestor and approval and I suppose the associated costs have been considered by the operators and rejected.
Regarding SBAS, the basic procedure design criteria specifies the use of the barometric altimeter, not the GNSS system, to establish decision height. Whilst GNSS is very accurate inthe horizontal plane, this is not the same in the vertical.