As for other fiat service and stablecoin, i would guess it's because probably Visa know some users only care about make payment or sending fiat (or it's equivalent) to another user at low fee or high convenience. Other fiat service might offer lower fee, while stablecoin on altcoin network have very low TX fee.
I am still a little bit confused with the fee and convenience factor, as the dashboard is only giving the stats about overall volume, or is it? Like the comparison of fees as well. Well, users definitely prefer lower fees and high convenience, they tend to easiness. They will adopt it and leave the other. I thought they made this dashboard to show people and users that they have got more volume, they are being used mostly than other platforms.
Speaking of the tx fee, I think it's not the only factor that's been considered by users, as alts needed to be integrated as payment methods in their sites, or platforms where the customers want, and it's a little hard, while implementation of Visa is more easier and many users considered it because there are fewer people using crypto and more people using fiat. Overall, this fee thing did not make sense to me, if you could please shed more light on it.