Maybe if Apple wins their fight with Qualcomm we'll start seeing cellular connectivity in more of their products. I personally have no interest in a cellular-capable Apple Watch, but I would be very interested in a cellular-capable MacBook.
It's amazing how hard it is to find a workable cellular SoC. Wifi? ESP8266s are $2-4 a pop and the Pi Zero W is $10 if you can actually find one. Bluetooth? An NRF51822 will only set you back $3-5, and it has a nice and simple Cortex-M0+ for the logic. And the ESP32 has wifi and bluetooth on the same module. GSM? It's being phased out worldwide so modules are being offloaded very cheaply, if your project doesn't need to be connected for long into the future.
But 3G/LTE? It's easier to find an HDMI receiver that'll process HDCP. Nevermind something hand-solderable...
If you take a look at the prototype, it had a slide-out antenna on the side of the lid.
They could have done what they did with the iPhone (And what HTC did first on the M7), injected plastic isolating part of the casing as an antenna but that would have been aesthetically odd.