Why would axons be unimageable?

There's research on the translation process where cells are basically flash-frozen (to avoid water crystals), then imaged with cryoelectronmicroscopy / AFM etc. where they image the translation process (RNA to protein) in order to get snapshots and get a better understanding of how the folding proceeds and is aided.

If we can image sub-cellular features, what makes you believe we can't trace all the axons, dendrites and the synapses?

It seems more like a question of how to do it cost effectively at scale, not so much a question of "can we or not?".