Why Voynichese doesn't have cribs?

[Jorge_Stolfi]

The 'crib attack' as you say relies on a framework of conditions that must be met in order to be viable. I am sure there's more but here are a few.:

• 'Voynichese' encodes text
  
• 'Voynichese' encodes text in a way that at least somewhat preserves its original structure
  
• 'Voynichese' encodes meaningful text in a way that at least somewhat preserves its original structure
  

All statistical analyses of the text (like Zipf plots, word patterns, etc) weakly support these three statements, to the extent that they are not terribly wrong for natural languages.  Whereas the opposite of those claims would probably have shown up clearly in those statistics.  Like a very flat Zipf plot, much higher or lower entropy per word, much bigger or smaller lexicon size, etc.

• The text is connected to the imagery
  
• The text is connected to the imagery in a similar way a known or expected plaintext would be.
  

These may be true for some details of the imagery. 

Like, in the Bio section, I believe that only the organ-like things and some tubes and tubs are part of the meaningful contents.  Everything else, including the nymphs and big pools, is decoration, or at best figurative representations of abstract concepts like "humors".  The labels on the tubes may refer to them; the labels on the nymphs, boh, who knows...

Again, I believe that the drawings in the Zodiac section are 100% decoration, and the only meaningful contents of each page is the list of 15 or 30 labels -- which refer to 1-degree sectors of the Ecliptic.  EDIT: and also the text rings. The labels are not mentioned in the text because the latter has nothing to say about any specific sectors.  Just as the preface to train timetable may not mention any specific train.

• The author copied the imagery from a source understanding its original meaning
  
• The author copied the imagery from a source understanding and preserving its original meaning
  

The imagery was copied, yes -- but by a Scribe that, by all we know, was not the Author, and did not understand a word of the text.

All the best, --stolfi