their could be many reasons...
IIRC, its a little heavier than the 6b, nothing that couldnt be dealt with, but just one more obsticle
Another good reason, its a wet sleeve engine. If you see the that people put into their cooling systems and lack of maintenance, i would never want to put a wet sleeved engine into a mainstream vehicle, it would cause never ending issues.
They require at least some amount of care and maintenance, which usually never happens for most vehicles.
At the same timeline, they were able to just use the 'mighty V8', which weighed less, had less potential for maintenance issues (in regards to being a parent bore block) and still produce comparable power.
2007 Laramie 3500, 4.5" RevTek, 35x12.5 ProComp MTRs, buckstop front+rear, bullydog TD, BD RFI intake, Lightforce 140s in the bumper, 240HIDs up top
1997 SLT 2500, 5.5" fabtech, 36x13.5 IROKs, 5x12s, s300, 4kgsk, inj pump FUBAR