The sulfates are likely cheaper than the chlorides. Plus the anhydrous chlorides (Ca and Mg) are difficult to keep anhydrous (esp if you imagine the amount they use) and the hydrated forms still absorb water making them difficult to pour (again on the scale they use). Sulfate salts don't do this as badly.
I believe at least one competitor product uses some potassium chloride as well.
Sulfate by itself has no smell - only when it is used as an oxygen source in an anaerobic (anoxic?) environment do you get H2S production - not something to worry about.
72g bowfront planted, CO2, 4x - T5HO, Eheim 2213 and 2217, 2 angels, pristella tetras, blue tetras, betta, albino bristlenose pleco, albino cories. Sword, vals, hygros, ludwigias, java moss and fern, anubias
2g Mac-quarium. Clown gravel, fluorescent plastic plants, and 2 guppies.