Chemically and for the home wine-maker: Cost and strength. Potassium (K) usually costs more than sodium (Na). If I recall rightly K is stronger, but I could be wrong.
Legally for the commercial wine-maker: Na cannot be used as an addition to wine because of the concerns regarding sodium in the diet.
Some folks use K for additions to their wine, and Na for sanitizing. Others us one for everything.
Steve