HDPE is the heaviest form of pond liner, and also the most chemical and damage resistant. Due to this fact, HDPE is often used with highly abrasive or corrosive chemicals, like those found in landfill leachate.
HDPE is also a more expensive option, partly due to its heaviness and difficulty to ship. Because it's so heavy, it must be cut and welded on site, rather than being manufactured in the warehouse ahead of time.