Different reasons could be what's written on your application, the type of site you have, the design of you website, the professional-ness of your website, your amount of traffic, your niche, etc.
Some networks do not accept incentivized type sites and then some do. Have you tried phoning them or tried joining up with some other networks also.
There is a list with reviews of many networks listed here:
http://www.affiliateseeking.com/netwo/23000002/1.html
Good luck Hanna1