I would guess the main reason is that the areas that would want to be in greater Serbia are so intertwined with the other areas that either the resultant countries would be so split up as to be impractical without full freedom of movement and employment between them and/or including many localities that would want to be in the other bit. Also that does provide ample incentive for ethnic cleansing, which do not go well last time.
Here is wikis ethnicity map:
Population density and ethnic distribution (2013 census data)
Here is wikis ethnicity map:

Population density and ethnic distribution (2013 census data)