[Invincible]

The U.S. government banned phosphates from household laundry detergents decades ago.  The new substitute is sodium carbonate and/or sodium borate. 

In the U.S. some dishwasher detergents are still formulated with phosphates(usually trisodium phosphate), however some states have banned phosphates from them as well and manufacturers not being eager to have extra SKUs, phosphate free detergents are becoming common in unregulated states as well.  Replacement products rely primarily on zeolite and sodium carbonate.

Why is the use of phosphates continued for dishwasher detergents? 

Environmental matters aside, what does TSP do that sodium carbonate, hydroxide or zeolite can't do cleaning wise?

Surely, if it had environmental consequences and there was a substitute with no compromise, they would have made the switch a long ago.  I believe sodium carbonate is a cheap industrial commodity as well. 

Phosphates are not banned here, but looking at older boxes of dishwasher detergents, I noticed that older ones usually say something like "this formula averages xx % phosphorus" while the newer ones say "this formula averages not more than xx % phosphorus"