Because, well, I don't even know how to explain...see, if you had just A, B, K and G (no A+B, B+K, no throws, no directional inputs, let's say bordering DK) you'd have three options of attacking. The opponent would know that that's all he'll have to deal with, pick up your possibilities rather quickly and adjust to them. Because: one option, one strategy. Adding directional inputs to that (especially 2k or other lows) will have your opponent guessing what your next move will be. And in some cases, he might guess wrong and take damage. Which opens up that thing called mindgame.
While the amount of moves in the current movesets of SCV are arguably still on a much bigger scale than in my stupid example above, the point remains essentially the same. Reduction of options equals reduction of mindgame potential equals reduction of strategies. On the other hand, you can have shitty strategy despite large amounts of options, true.