When the duo came out, very few stores in my area sold TG-16 games and those that did kept little stock.
The pack-in games, in my case, helped stave off the delay it took for the new releases and additional stock.
I also think that some of the advertisements used the retail value of the pack-in games to help offset the initial "high-cost" of the DUO. I figure that the pack-in HuCard was probably taken from overstock. Out of the 3 duo's I bought over the years, I think I got one Ninja Spirit, one Splatterhouse and a Dungeon Explorer 1.