The margin in the product itself may be consistent with other products... that doesn't negate it's other issues.... like size, low volume, and higher risk.
Since we are talking TRU here.. we are talking about retail. In retail, shelf space is an important commodity. Inventory is intended to CYCLE, not sit. Sitting inventory is cost as it's taxed and potentially depreciating. Unlike a private reseller, a retailer is not intesting in holding out and waiting to sell something. The space that sitting item is taking up is losing them money compared to that same space being used for something actively selling. And why retailers will aggressively put things on clearance.. instead of just 'waiting for a buyer'.
A set like 75192 is less desirable for retail because
It's extremely large - meaning it takes up expensive shelf and warehouse space
It's a price point completely different from your average shopper in that aisle meaning its difficult to promote and attract the right buyers
Its a low volume product. 10% on $800 may sound great... but 20% on $60 x 30 sure sounds alot better!!
Deadhead, damaged, or returns on such a high cost product are a huge liability
Often vendors diffuse these risks for retails by offering different programs where the retailer is almost like a consignment instead of just buying stock. TLG isn't going to make friends with it's retailers if it sticks them with open box or 'faulty' returns on high cost sets. TLG would also have to pay them a lot of money to get them to showcase these products because of the increased space these products need. Getting your product on the endcap with those big displays isn't something that just happens by chance
But the TL: DR is... high dollar, low volume, risky inventory is always less desirable vs high volume, easy to manage product.