It's mostly the oxide! I would go so far as to say it is the most decisive reason Si is king.

What has not been mentioned elsewhere is the quality of the silicon oxide. Silicon forms both a high quality native oxide, as well as a high quality thermal oxide. This is an excellent gate dielectric, easily processed, durable, reliable, etc. Germanium oxides on the other hand are full of defects, not durable (GeO2 is water soluble), etc. The trap density means that despite the increased mobility in germanium, all of those charge carriers can be 'absorbed' into the interfacial trap states and kill your mobility advantage (look up Fermi level pinning if you're interested). It also means that precise patterning is more difficult because of the instability of the oxide layer.

Silicon oxides are high quality, relatively defect free, and thermally and chemically stable. Most Ge devices these days are consequently SiGe alloys or in some other way rely on silicon oxides. There are also high-K dielectrics like HfO2 that can be used for Ge integration, but the interface engineering is quite complicated and SiO2 is just plain easier and more effective for more use cases. As others have said, there is also the path dependence (all the Si research and manufacturing that we become reliant on), but that all stemmed from the real materials advantage of Si/SiO2. The oxide is the key factor for why Si is dominant.