Airtel — mobile connection-conversion from postpaid to prepaid

Address:

Chennai, Tamil Nadu, 600042

I went to velachery airtel showroom for converting my postpaid to prepaid connection. The guy at the service center asked me to pay the pending amount along with extra amount which was rs.540/-. I paid upfront and he told the same sim will be changed to prepaid in 3 days. Nothing happened in 3 days, so i called the support and asked them the status, to which they said there is no such request placed!

Why are companies so unwilling to convert to prepaid.