The maximum growth rate of ice crystals will occur when the difference in saturation vapor pressure between the water and ice is the greatest. This difference is greatest when the air is in the temperature range where the dendrites are most likely to form. As the crystals fall through the various temperatures, the dendrites grow at the expense of the other kinds of crystals. The atmospheric conditions that produce sleet are such that falling snow travels down through a warm layer to melt into a raindrop and then through a deep freezing layer next to the ground and turn into ice pellets. This doesn't occur in warm weather obviously.
Hail forms in very unstable convective clouds such a cumulonimbus.
Sleet falls as small ice crystals and freezing rain falls as liquid drops and freezes on cold surfaces.