Because it's not... there's nothing on the roadmap, nothing has been announced and there's no plans to. Simple It couldn't come in the first place because it was so old and not compatible to work in tsw2.
Well it's a complete rebuild and there's still no plans for it. So you can only be patient and wait if it ever comes at all.
A complete rebuild of what exactly? Its not a locomotive. The only difference between it and most current freight cars is the 3d model and weight
Most US freight cars use standardized bogies, breaks, etc. From a code perspective it should be no different than a boxcar or anything else we get to drag around. There might be a few things they have to touch up on the model but not much. Like hours, not days.
Only real thing that would need to be addressed is livery editor compatibility, but not every old stock gets that either (Still waiting for the GP9RM to get it)