After all, a Nexus device from Google and whichever hardware manufacturer that they decide to partner with generally does not receive any input from mobile carriers, so T-Mobile was probably in the dark like you and me during the development of the Google Nexus 4. This means there was no input from T-Mobile for the Nexus 4’s manufacturing process, hence the lack of support right from the get go for such a feature. All in all, this should not come across as a surprise as it is a pure Google device, hence no carrier-provided software will be part of it.
Also, some folks did mention that WiFi calling smartphones do come with a specific kernels that was developed by the carriers themselves, and if you have a high level of technical knowledge, tinkering around with 3rd party kernels and custom ROMs, who knows, the Google Nexus 4 might just be able to handle WiFi calling after all.