My (admittedly limited) understanding is that one of the biggest issues being developed was how to get the critical mass together in the most efficient manner.
Once you get a sufficiently large enough quantity of fissionable material together it will start a chain reaction and begin converting its own matter into a large amount of energy. The more matter that is converted into energy, the bigger the explosion.
The problem is that the explosion that is occurring pushes the fissionable material apart and ends the chain reaction. So the scientists and engineers needed to develop a way to bring the critical mass together that was incredibly precise.