Old technology becomes obselete
Because technology advances fast, the technology of older phones becomes obsolete, and as a result, the phones become undesirable to buyers who want to buy the latest technology.
Big technology leaps
Technology sometimes leaps fast in a short period of time, and so old phones might become obsolete in shorter periods of time. This happened to ordinary phones when smartphones were first introduced.
Newer models are released often
Most smartphone manufacturers produce new models each year, and as a result, older models become obsolete.
People upgrade their phones often
Unlike cars or watches, people usually upgrade their phones more often. This makes an old phone much less desirable than a new one. (See Why smartphones are so popular?)
Initial loss of value is very high
Mobile phones lose value faster than cars. As soon as a mobile phone is used, it loses a large amount of value.
They are not designed to last for long
Many new smartphones are not durable enough to survive for years. This makes their older models undesirable.
Some brands lose value faster
iPhones were found to retain their value more than other brands including Android. The strong apple brand helps the phone lose value with a slower rate. (See Why do some people love iPhones that much?)