I think timing. The HomePlug people released "HomePlug Green PHY" standard in 2010 which was a lower power lower bandwidth variant for "Smart Energy and Smart Grid applications". Fits the bill exactly! However this came not long before the first wave of affordable wireless technologies that fill the same niche and the subsequent explosion of interest in "Smart Home" IoT. I think even HomePlug kind of teamed up with ZigBee in the end: https://sunspec.org/wp-content/uploads/2015/06/Zigbee-SEP-2-docs-11-0167-18-seed-app-spec-draft-for-editors-review.pdf Whereas I think the HomePlug AV gear still solves a pre-existing more common problem (getting high-ish bandwidth networking across a gap that's infeasible for Wi-Fi.) HomePlug Green PHY did find widespread adoption in one other niche, though: the CCS2 charging standard for DC fast charge of electric vehicles!