Why did the railroad companies hire immigrants to build the first transcontinental railroad?

Mostly because there weren't enough willing Americans to do the job. Railroad construction was HARD labor, and after the Civil War most native-born people were looking to live a quiet, settled farming life.

Also, the desperate Irish and Chinese laborers would take more abuse than their American counterparts. They had come from lands where poverty and famine were normal and were happy to have steady work -- no matter how dangerous or hard it was. The Chinese were especially industrious and didn't drink, though opium abuse was an issue sometimes.