According to some sources, the purpose of MCAS on Boeing's 737 MAX variants is to increase the back-force needed to further raise the nose when flying manually at high angles of attack, in order to give the airplane acceptable handling characteristics when approaching a stall.
Both the MAX variants and their predecessors, however, already have an elevator Feel and Centering Unit (page 8), the purpose of which appears to be the generation of appropriate stick force feedback in all stages of flight. If so, then, at first sight, this would be the appropriate unit in which to implement the function of MCAS, raising the question of why MCAS would be the preferred solution.
A few possibilities have occurred to me, but they are just guesses:
MCAS functionality needs angle-of-attack input, which may not be available where the Feel and Centering Unit is located (in the tail), and it would be complicated to get that information to it.
This answer states the the Feel and Centering Unit is a mechanical computer; given that, it therefore might not be easily modified.
Modifying the Feel and Centering Unit would require re-certifying it in toto, not just its new feature.
In addition to modifying the handling characteristics, MCAS is also seen as contributing to stall prevention directly, by reducing the angle of attack.
NB: Recently, Dominic Gates wrote an informative article in the Seattle Times about the origins of MCAS.