Why does Zbrush not allow n-gons compared to other software?
I don’t know for certain but I think probably for two reasons:
ZBrush prefers quads. This is because quads provide an even mesh topology most suited for the sculpting and other operations that ZBrush is designed for.
ZBrush is designed to work with many millions of polygons. Restricting the number of points per polygon to three or four means that the best use is made of the system memory available and processing time is minimised.