# Why are protic acids not used for the formation of thioacetals?

The standard procedure for protecting a carbonyl group as an acetal uses dry acid, such as $\ce{TsOH}$, but when forming a thioacetal, the Lewis acid $\ce{BF3}$ seems to be the catalyst of choice. For example, see Corey and Seebach's original demonstration of dithiane acidity: Org. Synth. 1970, 50, 72.

Is there any particular theoretical or practical reason why this is the case? I am guessing that the boron trifluoride coordinates exclusively to the harder oxygen atom, making it a better leaving group and hence favouring the formation of products (since the reaction is under thermodynamic control). However, I don't see any reason why acid wouldn't be able to do the job equally well.