Why do external drives fail?

There are a lot of causes why external drives fail. Some widespread of these causes are

External drives were designed to move them from a computer to a computer, and are thus more prone to careless handling. You can accidently drop a drive or hit it against something, causing a mechanical disk failure.
Usually people don't use "safely remove hardware" feature. This can lead to a filesystem failure.
In devices with external power, power supply units (PSUs) do not provide a proper current because for inexpensive external drives PSUs are supplied of already poor quality. The power supply can degrade completely within a couple of years.
An external drive or its power supply can fail due to improper voltage of a grid power. This is exacerbated by the inability of the cheap power supply to compensate for a bad grid power quality.