First of all, it was the Macedonians, who used the "Phalanx" to it's full with their Pezhetaroi, and other types of Sarissa carrying infantry.
The Greeks "Hoplite" didn't disappear, Roman infantry had Spear/Shielders on the field, and guess what was the most used "loadout" in the middle ages. The spear, and the shield. Anyway the Greeks also had to adapt to the Sarissae using Macedonians at some point.
The "loss" of this particular style of fighting could've happened because of two things. First, the Roman way of fighting just made phalanxes obsolete, so it wasn't as useful after that. One could also argue that many re-discoveries happened during the dark ages and after, because of the "barbarians" destroyed Rome, Europe went back centuries in terms of technology and culture.
Then, during the supremacy of the knight, these long spear formations became a pretty good choice, considering they could negate charges easily, the pike was the deciding factor on the battlefields at this point, until Gunpowder became too widespread and popular.
The lack of shields.. well I'd say that they went for more mobility instead of ranged defense. Very odd thing when archery and guns were all over the place, but shields were almost useless at the time period pikes were re-discovered and used en-masse.
Keep it in mind that this is my opinion and I used no website to confirm anything. So feel free to correct me, Must've said something stupid along these lines.
Ah, about to their differences. Well, the Macedonian Phalanx was HUGE. I remember reading about it and it was a MANY miles long line, like a moving frontline armed with 3-5 meter long spears and shields.
The Swiss and others like the Tercio, used other methods like Pike squares, and mixing firearms and pikes, covering the gunners while reloading, etc.