The problem is when Capcom made Third Strike, they put alot of time and money into it( Third Strike is the most expensive 2-D fighter ever)
But, it did not profit as anticipated, and pretty much flopped.Capcom realised 2-D fighters were not as big as before and If they did make another Street Fighter, it will not be generate as much profit as it used too. Street Fighter 2 is the highest selling Street Fighter ever, and Capcom has yet to make a Street Fighter that tops it.
Also, if they make another Street Fighter it will most likely be in 3-D.