Overlap Capicatance and Silicon Gate:
Yes reduced overlap capacitance of silicon gate was the biggest reason which could be understood by circuit and system designers.
There were other process approaches which tried to replicate the self alignment and low overlap capacitance feature of silicon gates,ie SATO from TI(Self Aligned Thick oxide) and Refractory Gate MOS ( Molybdnum or Tungsten Gate), but Silicon Gate ended up being copied by everyone. Refractory gate materials were harder to handle and process in volume. Silicon Gate ended up being better because silicon could withstand much high temperatures than aluminum, after gate deposition. This allowed better contaminant removal and other beneficial post gate deposition process steps which could be performed at temperatures which would destroy the aluminum gates.
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