Are we talking the "city limits" or the "metro area". If were talking the metro area I think Cincinnati is doing fine. Within the next decade I think that Dayton will be included in the "Cincinnati Metro Area" similar to that of Dallas/Ft. Worth.
If we talking about the city limits there are a number of reasons the city isn't growing. You can first and foremost look at the dynamic of city politics. You have both Hamilton County and The City of Cincinnati trying to rule over the same area. You need an agreement on both in order to do a lot of things. One needs to look no further than the disaster known as the Banks from 2003 until 2011. Politics and bickering really held of the development of the most valuable piece of real estate in the city and something that has turned into a very good thing.
The City has poorer schools and higher taxes than most of the burbs. When my wife and I were looking for a house we chose our area because of the nice school district as well as no local income tax. Living in the city would force me to pay an extra 2% in income tax that living outside of it. As talked about in a different thread if I could get my kids into the German Language School and then Walnut Hills I would be fine with CPS schools, but outside of that the CPS schools are a disappointment.
In general I think Midwesterners like to drive. Unfortunately I think the boat on mass transit was missed in the early part of the 20th century when Cincinnati decided to quit building the subway. Today a massive project like that will be very expensive and very time consuming. I enjoy mass transit in areas where it convenient and successful, not Chicago. I "like" the idea of a street car but think the process has been muddled and is off to an unsuccessful start.