Why is silicon the preferred choice and not germanium for diodes, transistors & semiconductors?

Both are in use. However, one is more abundant than other and there are various reasons for that.
Previously, when the so-called nonlinear devices were discovered, germanium was the preferred choice. Reasons - High abundance, low purification cost and easily adjustable to the fabrication process of that era. The one thing that didn't work out well for germanium is its temperature sensitivity.
Silicon, on the other hand, was more stable. Cost wise, if we see, Silicon is more abundant on earth and after the invention of the wafer and sophisticated fabrication technologies, the limits that blocked the path of silicon to flourish as a leading semiconductor material in non-linear devices were abolished.
Silicon leads the way in many semiconductor devices and IC's. However, when it comes to low-cost solutions you can still find traces of Germanium based devices. Apart from temperature sensitivity, the evolution of VLSI and fabrication technology has played a major role in crowning Silicon as the master of the field but I guess its the efficiency and the CMOS compatibility that played a major role.
But present scenario is very different if you see it from a researcher's perspective. As we are going towards speed and energy efficiency, hybrid semiconductors are now being used as the base materials and many other materials have become the part of this game. Take a quick look at some Silicon Photonics or Plasmonic based devices and you will understand what I mean to say.

Regarding stability, I found out that Silicon is thermally more stable at higher temperatures since it's reverse leakage current is less(~nA) which is much less than that of Germanium(~mA). And we know that reverse leakage current depends on thermally generated minority carriers.