Since the Dragon craft is already pressurized why is taking it so long to get it approved to carry a crew?
Human-rating a rocket is a long, expensive process
See this questions for more informations What exactly does it mean to human-rate a rocket? As opposed to the escape system?
Every single design aspect and component of your rocket will be studied and need to be approved for human launches.
The rocket must also pass more stringent requirements than a non human-rated one, and some safety features such as a launch escape system must be added.
Humans need a bit more than just a pressurized container
Humans, as opposed to cargo, also need a life support system
After all, they produce large quantities of CO2, heat, various fluids, ...
And not only you need a system to deal with this, but you also need to certify it, provide backups, ...
Dragon did carry animals, but they were considered expendable and very low maintenance compare to humans (mice, flies, etc.)
Overall, live cargo sent to the ISS is about 1KG via Dragon, and it comes with its own self contained life support system.
- In order to live inside spacecraft you need not only pressure, but also enough O2 and remove CO2. All human support system should be duplicated. NASA tells what is risk off systems crash that should be achieved (number of failures per 100 flights or something like this)
- All systems should work in every flight scenario - rocket explosion on every flight stage, can't dock to station, can't deorbit in desired time because of storm on the ocean.
- Risk of fatal failure in case of rocket explosion should be not more than X on 100 flights, and SpaceX should prove those numbers
- Ability to reach astronauts in storming sea, find the capsule if the splashdown place is different from planned 5 etc ...
For details see on NASA site "Human-Rating Requirements for Space Systems"