The WiFi silicon vendors seem to base their SDK/reference software on OpenWRT (albeit often badly-outdated versions), so almost everyone selling a WiFi box ends up using some variant or fork of OpenWRT. It's been a long time since the days of Linksys trying to cut DRAM costs by using something other than Linux. There are still exceptions like MikroTik where the OS and configuration tools are a main selling point (still Linux-based, but not OpenWRT).