Lower frequencies lose less energy over distance and diffract around obstacles better than 2.4 GHz, so a sub-GHz radio with the same power reaches farther and penetrates walls, foliage and floors more reliably. These bands are also less congested, because Wi-Fi and Bluetooth do not use them. That combination is why LoRa, Sigfox, Z-Wave, Wi-SUN and Wi-Fi HaLow all live below 1 GHz.
There are trade-offs. The available bandwidth is narrower, so data rates are low; the antenna must be physically larger to be efficient, which complicates tiny devices; and the exact band and rules differ by region, so a product sold in Europe at 868 MHz needs a different variant for North America at 915 MHz. Regulators often impose duty cycle limits in these ISM bands.
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