1 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.
2 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.
3 Newer models are released often
Most smartphone manufacturers produce new models each year, and as a result, older models become obsolete.
4 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?)
5 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.
6 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.
7 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?)