There are a lot of benefits and reasons for this.
1. Many people have a merchant account already with Authorize.net and would like to keep all the transactions in one system (for easier accounting and tracking fees).
2. You have the option to turn OFF monthly reminders when the subscription is about to renew. With paypal they send out a reminder every time before the subscription renews which encourages people to unsubscribe and annoys them by sending a bill. Makes people reconsider if your forum is worth the cost, so you have to re-sell them on the benefit of upgrading each time, otherwise they'll cancel.
3. Paypal owns YOUR data. They can cut you off at any time and TAKE all your revenue away. With Authorize.net you are in control of your own data so can move to another payment processor if need be.
4. The merchant account fees are usually lower than paypal so you save money.
5. You could also use Authorize.net with a future Xenforo-based shopping cart system (and it may be easier to integrate, more flexible than paypal). Or possibly even integrate Xenforo with an outside shopping cart system.
6. Customers can easily update their accounts through the Authorize.net API and change credit cards, pause subscriptions, change email/contact information and cancel account. With paypal, doing any of these things is long and complicated and not user friendly. These features could all be implemented from within the Xenforo member control panel. Also cuts down on admin having to manually change or edit these things. Or hiring someone to do this.
*** Keep in mind that credit card transactions and card numbers DO NOT have to be stored within Xenforo, so there are no issues here with PCI compliance. You can use Authorize.net's recurring billing system and API which submits the information to them and then they store it on THEIR servers. I think there is a $10/month fee they charge, but they also handle all of the customer information and follow-up rebilling (retrying bounced cards, etc.).
More information can be found here about Authorize.net's Automated Recurring Billing (ARB), along with a video demo etc.
Case Study #1:
Case Study #2:
API and Developer information:
I don't know what is involved in adding another payment option, but it seems that this would add a LOT of value to Xenforo with very little development time to implement. The API seems well documented and there is sufficient demand out there for other payment option choices besides Paypal.
If this is something you want, then please LIKE THIS POST and vote.