Simply because neither the TM1000 module nor the ARXXXT telemetry receivers are able to directly decode and interpret the telemetry data transmitted by modern ESCs and the iKON2 can actually receive this ESC data using a single wire between it and the ESC. This translates the telemetry and then sends it to the SPMXXXX receivers on the same bus from which it receives the radio signals. This functionality is not available with the older telemetry modules/receivers.
Using this method saves you from purchasing additional hardware to sense current, voltage, temperature, and RPM. Those purchases can be expensive and will result in added weight on the model.
Additionally, ARxxxxT receivers have unidirectional communication between receiver and iKON2 and are unable to take advantage of the Spektrum Integration to tune the model remotely.
Another reason is the redundancy of physical connections. With SPM receivers and two satellites connected to the iKON2, there are three physical connections (cables/connectors) and if one or even two of the three connections fail you are not at risk of losing control of the model.
With satellites connected to ARxxxxT receivers the connection is unique and if even one of the contacts of the two connectors or even one of the three wires fail or shorts against a corner of the carbon frame, you would have the total loss of control of the model.