I’m not entirely sure, but I think the answer is: four is enough! In fact, two is enough: just look at us! (And at ostriches 😉 )
Why isn’t four enough for insects? Well, I can’t say for certain, but I think it’s because of two reasons:
1) Insects (like ants) *always* have three legs on the floor – they take steps with three legs at a time, while standing on the other three! This means they cannot fall over 🙂 So, walking is simpler for them!
2) Because they are smaller, and compared to us, their “bones” are stronger, their legs can be smaller than ours, even relative to our bodies! So it doesn’t cost as much energy and food grow 2 extra legs 🙂
It is just the way we have evolved! As we are bigger it is less feasible for us to sustain more limbs… Four seems to ideal for large species, where as a lot of small organisms have multiple legs. Had 6 limbs of been sustainable and required, we would have evolved to have them 🙂
Hope that helps answer your question! Don’t forget to vote 🙂