In explaining this phenomenon, I will use the Darwinian approach. Tali, on at least one occasion, has mentioned evolution. Geth may have their own version of natural selection, where the geth network is the selector. For example, Hoppers have less armor than other Geth. The trade off is speed. But is that speed useful if they can still be gunned down? Geth would more than likely deem them to be inefficient given their purpose, which is to avoid fire and continuously disorient the enemy while conducting hit and run tactics. If a unit fails at its purpose, it becomes obsolete. Same goes for Juggernaut. With gigantic Primes controlling the field and smaller geth and rocket troops carrying out the tasks, is a middleground juggernaut necessary? Due to a logic driven nature, Geth would probably say no. Consider H2O. As a liquid, we drink it. As a solid, we use it to cool things. Then theres the middle, which is slush. Do we use or need slush? No. Same case for juggernauts. They are similar to primes in size and rocket troopers in function. Being that a rocket trooper already carries out that function and the prime fills the big geth niche, there is no need for the juggernaut. Even now, we see new appearances. This is not just to freshen up the game. It can be explained through Geth logic. Geth are very function oriented. If an upgrade to the Geth Prime allows it to do its job better, then it makes sense that the Prime is given its upgrade and the old variant of the prime is no longer produced, which is illustrated through its change in appearance and arsenal in Mass Effect 3. Geth are always evolving and doing so much more quickly than any other race. It is no surprise that weaker or less effective variants of Geth are fading out while stronger and improved variants are coming to the surface. And when these variants are no longer as effective as another new form that may arise, they will fade out and the Geth will once more take on new forms. So why did the Hoppers and Juggernauts fade out? Evolution and Natural(Geth) selection.