The Voynich theory I find most compelling is that it was a hoax made for a quack doctor, made to look like a foreign herbal manuscript. "Oh of course the local doctors can't help you, but my special book from a faraway land that only I can read may have the cure." Some recent analysis of the manuscript has found that the pages are more linguistically similar when read as individual flat sheets than how they're read when bound (i.e. whoever was writing the text was most likely going sheet by sheet and using the last completed page as reference for the text). The manuscript has only been bound once, in the fifteenth century (around when the vellum pages have been carbon-dated to), so whoever bound the manuscript was not able to "read" it. See https://journals.openedition.org/digitalmedievalist/2331