These conditions are apparently very rare and elusive.
They are not as rare as originally thought - significant densities of ice crystals near (tropical) convective storms appear to be relatively common.
The problem is that flying through these ice crystals does not always seem to have an effect.
Satellite and radiosonde data indicate that cloud tops in the area were higher than 50,000 ft. Perhaps they were entering a new cell that was just building up below them, and the particular ice particles are unique to that situation?
They flew through a large-scale system that was composed of 4 smaller cells. At the time they flew by it was beginning to diminish, not build.