Because of electronic cigarette e-liquid flavour concentrates are carried in Propylene Glycol (PG) in 99.99% of all cases, the industry adapted the use of PG to thin down Vegetable Glycerine (VG). VG is the main ingredient used to provide the plumes of smoke on exhale.
Because VG is too thick for any type of clearomiser, cartomiser, or any other delivery system, we use PG to thin this down. That basically the only reason we use these 2 ingredients.
And, because they are a food product.