Too widely used with too specific qualities that aren't closely enough replaceable. There are replacements for transfat that don't dramatically alter the taste or texture, but you can't drink a soda and get drunk.
It's politically a more difficult maneuver; we know because we tried it in the 20s
There are certain dishes, which require shortening, that there is no effective replacement such as Apple Pie or Southern style biscuits. Shortening creates a flakiness and funny enough in floured baked goods, creates a flavor more buttery than butter alone, it has a magnifying property. Anyways the loss of (trans-fat) shortening will / has certainly drastically alter certain types of quick bread baking.
Shortening's like Crisco have been reducing the amount of trans-fats and the results can be seen in the loss of flakiness and butteriness of quick bread baking so much so that there is already a market for the older stuff.
I find lard and butter together are often best. It's amusing that GP complained about shortening becoming less buttery. Hmmm, I wonder how one could fix that?
It's politically a more difficult maneuver; we know because we tried it in the 20s