Well, in addition to all of the reasons above, I can think of one other.
A slide capable film must be able to have all of the negative silver developed in the first developer or there will be film left to form a gray image in the highlight areas of he positive. If the film in question has a fair amount of undeveloped grains in normal reversal first developer, then those grains can develop in the second developer.
To combat this, a first developer is usually a "foggy" developer with a silver halide solvent present to force total development of silver in negative dmax areas. Then the second developer can go to completion without causing gray highlights.
If these conditions are not met, then the film is not a good candidate for making slides at all.
PE