The difference is attitude.
When the enthusiasts discover that some devices employ firmware is open source.
Most manufacturers of these devices ignore the 3rd party firmware.
Cisco (I.e. Linksys) changed the Firmware to propriety (WRT54Gv5 and above) and offered the Flashable WRT54GL for more money just because it is Flashable. They did not even attempt to spend few hours to improve the product themselves.
Buffalo (to the best of my knowledge) cooperated with DD-WRT requests for info in a guarded but reasonable way.
We do have to take into account that the trend started with the sv*** something (I forgot the exact name).
Sv*** practices where somewhat Bizarre, which probably encouraged some of the manufacturers to shy away from the issue.
To date No manufacturer, or 3rd party vendor, has an extensive easy to understand help as DD-WRT has on Wiki.