Let me argue that selling Article Publication Charges (APC’s) is, at its core, simpler than selling subscriptions. In order to do this I need to create a hypothetical journal.

The Hypothetical Journal
THJ Publishes 50 papers per year
If THJ were an OA journal it would have an APC of $ 1.000 per paper
If THJ were an OA journal it would have an institutional subscription price of $ 250

THJ as an Author Pays publication
If THJ would publish in OA it would have revenues of $ 50.000 (50 papers multiplied by the APC of $ 1.000). You would need to send each of the corresponding authors an invoice. So 50 invoices in total and you would know where to send these invoices.

THJ as a Reader Pays publication
In order to have the same revenues as in an OA journal you would have to sell 200 subscriptions (200 subscriptions of $250 each make also $ 50.000). This means that you have to send out 4 times as many invoices. Now sending out invoices is not that difficult. The problem is that you do not know where to send them. Who are your subscribers??? Say that 10% of the people you try to sell a subscription actually buys a subscription. That would mean that you would have to approach 2,000 potential subscribers. That is quite a lot. And, let’s be honest; a 10% success rate is pretty optimistic. Apart from that; who are the 2,000 potential subscribers?

Conclusion
Selling OA is, at its core, simpler: you have to make fewer sales and, each accepted submission is a sale!