When you freeze tissue, ice crystals damage the cells as they grow. If you can get tissue way below freezing very quickly, many small ice crystals form instead of a few large ones, and no individual small crystal is forcefully pushing its sharp edges into bits of the cell.

By the square-cube law, it's easier to rapidly cool a small thing than a big one. Cells are much smaller than tissues made of many cells, so you can freeze them faster and get less damage.