I guess it is the location which is closely associated. Tata Steel is located in JSR. Train also originated here and has been a prestigious one over so many years hence Steel getting the name.
Ispat express was named more from the point of view of SAIL being located at ROU.
Even the Tata-allp/DHN-Allp earlier as running as Tata-Madras/Bokaro-Madras was written as BKSC-Madras steel city express as it was originating in BKSC. Remember seeing some old TT with the same being mentioned