Answer

There are several reasons at it depends on the device.

For some devices like the unD4O, the unDUSB, and the D2FLEXIO, they don't have any user settings (the D2FLEXIO actually does but they are all mechanical). For those devices, there is nothing for a plugin or extension to actually control.

There are some devices that have an extension but no plugin. Devices that have plugins were all released prior to extensions becoming available. The DBU, BT1, USB1, D2I, and DTH1620 were all released when extensions became available. Extensions can do everything a plugin can, but with the added benefits that they can be used with Reflect and do not requiring a scripting license.