Exactly what George mentioned, as was very well explained in Bob Zenk's book. But possibly the most significant point was horsepower. Domestic builders weren't offering much in the way of increased HP, so SP & DRGW looked elsewhere. The K-M's made their case that higher HP was possible. This got the attention of US builders who soon embarked on producing 2nd generation locomotives with proven, and improving, electrical transmissions. The hydraulics won the battle but lost the war.