What could cause a hypolastic kidney (only one kidney)?

Single kidney. There are several reasons why people have a single kidney. Often during development before we are born, one of the kidneys can fail to form (renal agenesis). This can occur for many reasons (issues with the ureteral bud or mesonephric ducts, etc). There can be renal dysplasia, where the kidney just does not form quite right. Also severe renal artery stenosis can result in renal dysplasia.