Forgive my ignorance, but why are ITBs falling to the wayside? Seems like they would ride better than ATBs and maybe even a little cheaper to build. What am I missing?

Look at NVIC 2-81. There are 2 types of ITBs; Pushing Mode ITBs and Dual Mode ITBs. With regards to Pushing Mode ITBs, those regulations and statutes applicable to a ship of the same tonnage as the aggregate tonnage of the combined units shall be applied. As most ITBs built were Pushing Mode type ITBs they are treated as ships and not 2 separate vessels. They lost the regulatory loophole that ATB enjoy.

Ahh, well that explains my question. Thank you for that.