Because their network was designed to offer the coverage they need with the 2100 MmHz characteristics. That's because this is the frequencies they are authorised to use. This also means they probably have a lot more 2100 MHz cell sites than Telecom.

Telecom on the other hand has designed their network around 850 MHz mainly. Because they do have access to a limited 2100 MHz range they decided to use it as infill.

This has been discussed many times, since the XT launch.