Imagine someone who's really bad at playing Tetris. They don't even try to make the blocks fit together well and they make lots of big empty holes in their pile of blocks as the blocks get stuck on each other in the least compact way possible. That makes their pile get high very quickly.

That's what a pile of snow is like. Raindrops are excellent at playing Tetris because liquids will change shape as needed to fill the lowest possible level of the container. Snowflakes on the other hand won't. Their very spread-out shapes, fully of spiky bits at the ends, cannot tessellate.

Snow depth is made mostly of the air gaps in between the flakes that get hung up on each other's spikes.