The simplistic (though not very satisfying) answer is … chemistry. While e-juice is really just a mixture of liquids (food grade flavorings and glycerin plus a little nicotine), that little bit of nicotine is actually fairly important. (I may be wrong, but isn’t it one of the reason for vaping?)
First let’s talk about what diacetyl is and why it is of interest to anyone. Diacetyl (fyi, I pronounce it as di-ah-SEE-till, but there are other ways to say it) and it’s relatives, such as acetyl propionyl, belong to the family that chemists call alpha-diketones. These compounds have the characteristic of imparting a buttery sensation to foods. They are used in a wide variety of products such as baked goods (cakes and cookies), confections (candy and chocolates), and snacks (potato or corn chips).