The DD patch is much smaller than the ongoing patches for D:OS, and leaving it available doesn't involve an extra step each update to create and test a patch.
Dragon Commander was available from the Larian Vault, as well as GOG and Steam, so Larian knows what it costs (time and bandwidth) to have a separate patch process, as well as the relative demand for each version.
There was never a version of D:OS available directly from the Vault, and the demand for a patch is much, much smaller.
