Because they had it before, also with low floor trams it enables a wider ailse. Would the old system have been converted to standard gauge if it had surivied? If you are retruing streetcars to a city that had them before, I would not call that starting from scratch.
My question is, why not use a different gauge from standard if this system is physically separate from existing standard guage lines? If they were to choose the Pennsylvania trolley gauge, they would be bulit to the same gauge as both the prevouis system in Cincinatti and suriving networks in Pittsburgh and Phillidelpiha, with added benefits in case of low floor trams.
I realise that Cincinatti has chosen CAF's urbos, this is 100% low floor, but it is a fixed bogie design, and
we've been at length on this before, fixed bogie trams are no good on tight curves, they may not have clearence problems, but having entire carbody sections tracking curves greatly increases impact on tight curves.
Skoda's 15t would have been a better choice, it has 100% low floor and pivoting bogies. With a wider track gauge, there will be more space between the wheels relative to the aisle, reducing constraints on bogie movement. Also if made for a North American system, the North American versions would be wider than those in Prague and Riga, allowing for 2+2 seating, and longer entrance ramps so the entrance height can be lower, combine that with height adjustable suspension, and such a tram could provide easy access from classic style tram stops, making it very suitable for the street enviroment.