I'm sure we all remember the graph showing angle of attack against lift with the stall occurring at about 15 degrees. Fairly true for a straight winged light aircraft and lowish speeds and maybe not too far out for a 737 at 2000'. The same graph for a (slightly) swept wing aircraft flying at altitude above 0.7M would look completely different.
So presumably MCAS has a set of stored parameters as to what is an acceptable AOA in both regimes. It might allow say 14 degrees at 2000' but only 6 degrees at FL 390. Hence the need for inputs other than AOA.