That would not be a reason. If you took apart a burner and a non burner and put the parts out they would look almost exactly the same. The only difference is the lazer on the burner is slight different to account for the burner. Hardware wise they are exactly the same.Maybe the reason for the adapter is because these drives are burners as well. Perhaps the reason it works on the Wii is because it had the parts to make it work properly without jamming, but with the additional hardware for the drive to be a cd/dvd burner it is not possible.
You could lay out the parts and at most you would only see a different lazer but chances are they would look exactly the same.
Personally I am not a fan of slot loaders. I like the old fashion try loads. They are simpler and have first parts and is less likely to break. They are cheaper as well and as an added bonus they can be used a cup holder.