For ANC to be most effective the microphones should be at your ear. This makes more sense when everything else is at your ear too.

Also, the varying input power and other processing of audio in your source with an analogue output for IEMs (and if you have detachable cables, differences in frequency response and impedance in different IEMs) all would make the calculations of the inverse waveform much more difficult, if not impossible.

Within Bluetooth, everything is self-contained in the headphone or earbud. The processor knows exactly the impedance of the driver, the frequency response and volume. Any processing is passed to the headphone digitally so it can be accounted for in the digital to analogue conversion. And since everything is self contained the microphones can be placed as close to your ear as possible.