When we launched Rewards (then called 'Payments'), we did so using Bitcoin. During that time, our users would seek to buy about $5 or $10 of BTC at a time. Due to network fees and congestion, these users would pay as much as 90% to fees, and still not be sure when their coins would arrive. This clearly wasn't good for the health and well-being of Brave and Brave "Payments" (now "Rewards").
Our hand was forced, effectively. We needed to pivot to a solution that offered better throughput, with lower fees, or possibly die an early death as a project. Hopping to an ERC-20 token offered immediate relief in both areas, ensuring Brave "Payments" would be able to go on and develop.