The cause is primarily because hardware manufacturers of the devices are not able to upgrade to the later operating system for a number of reasons.
[5] This variable represents the device model, though it may be altered by adding a custom ROM.
OpenSignal acknowledged that while this made it problematic to develop apps, the wide variety of models allows Android to enter more markets.
[4] For example, OpenSignal and Testdroid studies have found out that OEMs fragment Android ecosystem significantly more than Google.
You agree that you will not take any actions that may cause or result in the fragmentation of Android, including but not limited to distributing, participating in the creation of, or promoting in any way a software development kit derived from the SDK.