We have to take address information in order to be able to work out postage. So you would not be able proceed to the payment stage without having entered your address information.
We have also found that all errors that arise in the use of our website are generally due to the transfer of information between ourselves and third party applications such as Paypal and the credit card service. The more information transfers the greater the probability of errors. So we prefer to minimise this by keeping paypal and all payment options to payments and restricting further information transfers.
Finally we also offer customers a couple of payment choices so that people can use credit cards or paypal. The choice doesn't arise until after registration. So having paypal only choice is not something we would want to implement.