Is it easier to make a sanitary napkin instead of a disposable diaper?

That depends on the features of each product.  For example, if you are comparing the most sophisticated sanitary napkin with the most economic diaper, perhaps both are as simple; or as complex.  In general terms, when we are talking about similar market segments, a sanitary napkin is much easier to manufacture than a diaper because it does not need as many features.   This is the main reason why a diaper producer can get into the sanitary napkins business easier than the other way around.