While most NASCAR races are on ovals, they have had a couple road course races in the past. I think they used only do 2 (Watkins Glen and another one), but now are doing more. However, those were always race tracks, this is their first actual street circuit.
I think NASCAR has been losing viewership over the last few years and is trying to turn it around, especially with F1 now making inroads into the US that they hadn't been able to before, with now 3(?) races in the US (Miami, COTA in Texas, and Las Vegas). NASCAR last year and this year did a short track race at the Coliseum in LA and this seems like their next step in trying to grow audience in areas they haven't been able to.