Why HYIP program doesn't not allow using paypal as a payment processor ?
All HYIP program support e-gold , is there any different between e-gold and paypal in HYIP program ?

Because Paypal will freeze the assets of the HYIP, since 100% of them are illegal ponzi scams, plus, when a person realizes that they have been taken they can not do a charge back with e-gold. Also, there is the seasoned playa that will get to making a profit, then, just before the inevitable collapse of the scam will do a charge back on their initial funds.

Stay away from HYIPs is the best recommendation anyone can give you.