Ksp stands for solubility product, which means that it tells how much of the item will dissolve (from Wikipedia, the example being Sucrose going form solid to aqueous in water) or dissociate (the example we have here) into the solution, with the rest being solid, so we leave the solid out of the equation and we have just the two particles in solution.
Ka on the other hand is the acidity constant, and I believe this implies that generally the acid is Disolved (e.g. HCl(aq)) and the dissolved product dissociates into H+ and Cl-. So because all three are aqueous we have them all in the equation.
Im not sure, but I was just thinking that if there were a compound that is acidic, but not very soluble, it would probably use a Ksp, since even though it is an acid, the solid is not soluble - Im not even sure if something like this exists, but I think my reasoning is correct.
Hope this helps.
