The main reason for this is to minimize my payment processing fees. As of this writing, PayPal is the only payment processor to offer a micropayments rate. This rate is much more fair to people who sell low-cost items, such as £0.49 subscriptions. If I were taking debit card payments for this amount, my fees would be much higher. PayPal is what makes such a low-cost subscription even possible. Without PayPal, I'd have to sell only the longer-commitment subscriptions, and I like my product being so accessible at a low price.

Fortunately, PayPal is also very convenient to use, and you don't even need an account with them. However, if you do have an account with them, you can pay with PayPal One-Touch which is even simpler. I feel it's a good option.