Why use ThriveC when I could just reduce the amount of Thrive or Thrive+ that I dose to limit nitrate levels?

You certainly could just dose less Thrive or Thrive+ to help with elevated nitrate levels, however when doing this you are also reducing the levels of all the other nutrients. Nutrients such as iron and potassium as well as a few other micronutrients are not commonly supplied by fish waste so dosing a well rounded fertilizer is important in these tanks. ThriveC does just that, it has lower levels of nitrogen as well as phosphorous (which can also creep high in heavyily stocking tanks), while still providing adequate levels of potassium, iron and other important micronutrients.