Apart from the comments given by other members, I would say it's a Pandora's Box. There are multiple reasons for a candidate to reject an offer. The main challenge I see is that any candidate, after getting his or her first offer, will be very confident, and their performance in other interviews would drastically improve. Also, being human, they tend to take chances by attending multiple interviews simultaneously.
Nowadays, I see many candidates wanting to join startups. The main two reasons I see are the salary package and the culture of the organization. If you ask a candidate to name 20 startups, how many would be able to apart from Flipkart, Mu-Sigma, Snapdeal, Myntra, etc.? Bangalore, India, has the maximum number of startups (more than 17,000) in the world, followed by New York, US.
Regards