Well, Shanghai has some, Japan is building some, and if this discovery is real, Germany might as well revive their Transrapid project (which built the shanghai one).
I guess that's partially missing the point. The main benefit of a superconducting maglev is that it's cheaper to build than a on-rails high-speed train. Specially on problematic terrain (what I understand includes every possible East/West route on the US).
Why would it be cheaper? The actual rails is a pretty small part of constructing high speed rail, isn't it? Especially on problematic terrain.
Energy usage and passenger bandwidth could be a reason
Because maglev trains can make stronger curves and climb steeper hills.