That is sort of the point. And it is what regional play is for. If every city can do everything, or almost everything, then the concept of the region falls apart. Some of the smaller regions could likely still do fine with larger city maps. A casino and tourist city can bring in a lot of cash but it relies more on the region then other specializations. Without plenty of sims in your region there won't be that many tourists.
For example I started a public region and am financing my city off of coal,ore, and oil exports. My city would have failed without that steady income coming in and I can even operate in the red for a while. The problem my Casino city is running into is the lack of workers and the lack of Sims to gamble. Oh I can get sims to the casinos but anything other then the gambling hall isn't making a profit.
What I really need is to have a few other cities in the region to supply me with tourists and workers. I have plenty of jobs to be filled. It would also allow me to cut back on my residential space opening up room for airports, landmarks and other tourist things to draw them in. I could also greatly expand my commercial districts. But my region hasn't seen any one join it so I'm struggling to specialize.
With one city you can do everything fairly well, but you will never excel without the help and support multiple cities can provide. For example if you had a balanced city, a Casino city, and a Trade/Mineral city in one region each one could thrive. Mine/ore provides the money, balanced provides workers and city services, and casino provides tourists and services (as well as money when you can get enough people.)