The manufacturer usually modifies OpenWRT/QSDK to support their device. AFAIK, most of the time individual components from the device (CPU, Ethernet switch, wireless chip) are already supported in OpenWRT, it's just that the specific combo that the device contains just isn't there yet. This configuration is done with the device tree. On top of that, some manufacturers (Tp-Link, for example) don't use the standard OpenWRT sysupgrade image format, so the device rejects the new firmware that you try to flash.