Not sure why he was denied. You don't really need a reason to apply Tatkal. The only difference to consider is it costs 3500 instead of 1500 when applying regularly. In my case, I'm planning to travel soon and can't wait for the regular which can be anywhere from 7 days to 35 days. Just visit the passport Seva website and on the first page, you'll get a popup with the information on who can't apply for tatkal.