The turbos of today have much better aerodynamics combined with lighter wheels and higher flowing compressor covers.
Combine that with better boost controllers and suspension parts most people will tell you twin are not neccessary to get the turbo response and flow to make higher horsepower.
2 turbos 2 gates the room to install them with the weight over the nose plus they need to be controlled.
Its literally twice the expense.
Especially when 3 bolt flanges are good over 1000hp and 4 bolt flanges turbos are good for 1500+ horsepower.