This is a common lament of women. The answer is easy and simple to explain; but perhaps of more interest is the reason why women are left wondering why.
As to why he doesn’t call, the reason is the he suspects that she wants to take the next step in a relationship, which normally means more work for him for the same amount of sex. Probably the fella that she wishes would call is attractive and sexy in some manner, and has options. For these fellas, it is just a simple cost/benefit situation. If women really want a guy who will call back, they need to stop shooting for the sexy cad types.
As for why women wonder why, the answer is easy. Nobody tells them the true situation. Why is this? Because the situation is somewhat embarrassing to women. Due to hypergamy, women all flock to the sexy guys (many whom are cads), giving him lots of options. Furthermore, many perfectly acceptable fellas (by any objective measure) are left on the shelf. This is not a good look for women in general, hence it must never be mentioned.
Better to suffer the collateral damage upon both men and women rather than embarrass women as whole
Exit question — How much collateral damage because of this is there?