Why is WiFi often impractical for a coin-cell battery device?

Because the WiFi radio's power draw is orders of magnitude higher than BLE's, and the protocol's overheads compound it. Transmitting and even receiving/associating on WiFi pulls tens to hundreds of milliamps, while a coin cell supplies only a small capacity at a limited peak current, so a continuously-connected WiFi device would drain it in hours to days, not the months/years coin-cell products target. Duty-cycling (deep sleep, wake periodically to connect-send-sleep) helps, but each wake isn't free: re-associating with the access point, completing DHCP, and establishing a TLS session take time and energy every cycle, so the wake cost, not just the sleep current, dominates the energy budget, and frequent wakes erode the savings. Coin cells also struggle with WiFi's current spikes during transmit, which can sag the cell's voltage. So WiFi fits mains-powered devices, devices with larger rechargeable batteries, or ones that report very infrequently (where the long sleep amortizes the wake cost). For a sensor that must run years on a coin cell, BLE's tiny per-event energy and long sleeps are the right fit, and if that data must reach the cloud you bridge through a gateway rather than putting WiFi on the sensor itself.