Because you can do many more reps overall than just pumping out a few max sets. More reps = more volume, more volume = better results.

The idea with ladders is to keep the rests between sets very short. So in effect you're working out at very high intensity but sneak in just enough rest to avoid burning out. Also, the low-rep sets stimulate your neurological system. Your muscles "learn" how to perform a given action optimally. In turn, that leads to better performance and more strength (without necessarily adding extra muscle mass).

Look into Pavel's Power to the People if you want some more info. Ladders are pretty effective whether counting reps up or down.