The biggest reason is you have to show why they should host with you. I pay hostgator 100 bucks a year, for unlimited everything, my website has never been down, and i can talk to someone 247 through live chat and have never waited more than 3 mins for an operator. Can you compete with that? Probably not for the simple reason is unless you have enough money to rent out a data center and pay employees you will have to go with crappy reseller hosting and you can only offer so much, because they are probably reselling aswell LOL. But if you stick at it you can make some money.
If you are still keen then give hostnine.com a try. They offer unlimited everything reseller accounts plus free billing software and enom reseller account and a bunch of other freebies for 80 bucks a month.
I used their reseller services before and was quite happy.
P.S. Here is an idea of the competition. 99.9% of hosts that advertise their hosting on this website are resellers themselves.