what is IUPAC name of saccharin and why?

Answer 1

1,2-Benzisothiazol-3(2H)-one 1,1-dioxide

The pentagonal isothiazole molecule has S and N next to one another at two of the corners. The third corner is C=O (3-one). The hexagon of benzene shares a common side (positions 1 and 2) with isothiazole. If three consecutive corners of isothiazole are taken by S, N, and C=O, then that leaves only the side bounded by corners 4 and 5 to share with the benzene, so one need not specify them. 1,1-Dioxide denotes two =O bonds to S. In isothiazole, there would normally be a double bond involving the N at corner 2. There is not, so one writes (2H) to denote a hydrogen there instead.

Answer 2

o-Benzoic sulfimide
because it's the sulfimide (NHSO2R) derivative of benzoic acid

There are more exhaustive IUPAC names, but this one best permits an answer to "why"...