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Why were early transistors made from germanium?

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NMOS transistorNegative-channel metal-oxide semiconductors (NMOS) employ a positive secondary voltage to switch a shallow layer of p-type semiconductor material below the gate into n-type. For positive-channel metal-oxide semiconductors (PMOS), all these polarities are reversed. NMOS transistors are more expensive, but faster, than PMOS transistors. — transistor NMOS transistor
Negative-channel metal-oxide semiconductors (NMOS) employ a positive secondary voltage to switch a shallow layer of p-type semiconductor material below the gate into n-type. For positive-channel metal-oxide semiconductors (PMOS), all these polarities are reversed. NMOS transistors are more expensive, but faster, than PMOS transistors.

Early transistors were manufactured using germanium primarily because, during and shortly after World War II, scientists had already developed effective methods for purifying germanium to the extreme degree required for semiconductor devices.1

Because the electrical properties of semiconductors are highly sensitive to even trace amounts of impurities—tolerating only about one part per billion of foreign elements—the ability to achieve this level of purity was the deciding factor in its early use. While silicon eventually became the preferred material due to its greater abundance, lower cost, and superior performance at higher temperatures, it was initially much more difficult to work with than germanium.12