Why Voynichese doesn't have cribs?

[Jorge_Stolfi]

And going back to the lack of cribs I can see some other possibilities:

There is also 

4. The language is not any of your guesses

Again, no cypher is as hard to crack as a language that yo don't know.  By a long shot.  

In those examples you gave, the cribs helped only because the guy knew the underlying language, modulo a millennium or two of evolution, and guessed it right.  You can add Hittite (deciphered thank to a crib word "bread" and the guess that the language was Indo-European). 

But the crib will not help if the underlying language has a very different grammar from the languages you know, and very different ways of expressing concepts.  

Linguists have been studying the Etruscan language for more than a century, but their knowledge of it is still very limited.  In spite of hundreds (if not thousands) of "cribs".  Until recently they had identified the words for the numbers 1 to 10, but were still uncertain about 4 and 6: they knew the two words but did not know which was which.   That was solved only by a statistical analysis of the patterns of numbers in Etruscan dice, and the chance find of a pair of dice where the numbers were written as words rather than dots...

The omission of labels on figures copied from other books is expected if it they were copied only for their decorative value.  Imagine the Author telling the Scribe, "here, in the center of this diagram draw something like this" and gave him a book.  That book may have had labels; but since the Scribe did not know Voynichese, he would not know how to translate them. 

But there are many potential cribs on the VMS.  Every label is one.  We only to not know what it is that they are labeling...

There is a crib on page f68r3: the name for the Pleiades (doary, BEEPed as doaro).  Unfortunately it is not mentioned in the text.  Which is not unexpected, as it probably was irrelevant for whatever astrology is discussed in the book.

All the best, --stolfi