In order to make the nasal burn of horseradish more appealing, Arby’s added mayonnaise and sugar to the horseradish, blending recognizable flavors and novel zingy notes.
Horseradish isn't so expensive that it would be prohibitive to use in fast food; it is more the novelty of the flavor that makes Arby's twist on horseradish sauce so popular.
The sauce nowadays is made using horseradish powder, mustard flour, and vinegar. It’s a sweet, tangy, creamy sauce with a moderate kick that's delicious on sandwiches and as a dip.