Considering the car does not have Google's newer vector based maps, my guess is that it's a licensing/money thing. Tesla either has an existing, lower cost license, or maybe none is even required, since they're just using a web browser and it was the easiest way to throw up a big realtime map on the screen. But you can't get Google turn by turn directions through a web browser, so they would no doubt have to pay Google, or implement Car Play or Android Auto to get that. Hence, why for the instrument cluster and turn by turn directions, they use a more common auto map database.
On the positive side, so many cars have terrible systems, but NO other cars get over the air software updates where we might actually have a hope of getting something better in the future. Here's hoping they overhaul this one way or another with v8, presumably before or in time for the Model 3 launch. And if it comes sooner, all the better!