Reasons why an ESTA application is denied:
On a previous visit to the United States, you overstayed beyond the amount of time allowable for your visa or visa waiver.
On a previous visit to the United States, you did not have the appropriate visa to work
You previously made a request for a visa or visa waiver, and were denied entry into the United States.
The answers you provided on your ESTA application form were incorrect, once cross-checked with the US government systems.
You provided the passport details for a passport that you claimed to be lost or stolen, yet actually was still in your possession.
You have a criminal record, and regardless of your answer to question 2 on the ESTA application form, you were found to have a conviction history.
You have the name of someone who committed a crime in your name, or you are a victim of identity theft.
(But these are only the "most likely" reasons.)
www.official-esta.com/esta-resources/reasons-why-an-esta-application-gets-denied/