Bandiwdth, mostly. The lower you go in frequency, the less bandwidth is available. The entire "HF" spectrum is ~30 MHz. My Wi-Fi at home uses a wider channel

Bandiwdth, mostly. The lower you go in frequency, the less bandwidth is available. The entire "HF" spectrum is ~30 MHz. My Wi-Fi at home uses a wider channel than that. The lower frequencies -- that will travel across the ocean, unaided -- don't have anywhere near the bandwidth necessary (to support the throughput that they need).