Because the only 50wt motor oil you will find today is detergent motor oil.
In reality the dana 20 will survive a long and happy life with just about any oil. Just using normal gear oil keeps the oil inventory down. There are probably more transfer cases filled with gear oil from old service centers doing service jobs and filling with standard gear oil back in the day then you think there are. Every other dana 20 application is gear oil. Ford had some strange thing about filling manual transmissions with motor oil a long time ago.
Yea, I had the transmission worked on in a shop once and they had to pull the transf case. After that, I noticed shifting in and out of 4H was much more difficult when cold. I checked, and sure enough, they had filled it with gear lube. I had to show them the sticker on the door which showed the SAE 50/30 specification. When I asked the guy "why did you do that?" he said he had no SAE 50. Good thing he wasn't doing a tune-up. He probably would have used the wrong spark plugs because he didn't have the right ones.%)
You can't just use your gut feeling about what should go in a gear box. Chevy S-10 pickups took SAE 30 in their trannys and would shift terrible when gear oil was put in by mistake.
I can't find "non-detergent" in the Ford specs. I understand that detergent is there to keep engine contaminants in suspension, but the only contaminants that should occur in a transf case are metal particles and water. No detergent would keep either of those in suspension.
I'm no oil engineer. Way above my pay grade, so I feel more comfortable using the expert's recommendation.