Swiss meringue buttercream is made by heating a syrup of egg whites and sugar. The syrup is then cooled as it is beaten to stiff peaks, and then butter, salt and flavorings are gradually incorporated. It has a very smooth and silky consistency and is quite stable. Swiss meringue buttercream has less sugar than American buttercream, which is helpful here as the caramel sauce adds additional sweetness. American buttercream consists of mostly butter and powdered sugar, so it's already very sweet. Adding caramel sauce would increase the liquid content significantly, which would require adding even more powdered sugar to thicken it. You'd end up with a tooth-achingly sweet frosting that is grainy from the added powdered sugar. Additionally, it's much easier to incorporate a small amount of liquid flavorings into Swiss meringue buttercream. You can often add sauces or jams to Swiss meringue buttercream (like this salted caramel sauce) without adversely impacting the final consistency or needing to make additional ingredient adjustments.