In 1942, when the Manhattan Project began, there were many unknowns. They were not totally sure they could produce either form of material — both required technologies that frankly had never been developed at the industrial scale — and they didn't completely know whether either could be easily used in a workable bomb design. So they hedged their bets and developed two entirely different routes to a bomb, with the hope that one would pan out.

As it turned out, one of the materials — plutonium — couldn't be used in their original, simple bomb design. Reactor bred plutonium has impurities in it that would cause it to pre-detonate if used in a design like the Hiroshima bomb. So they then had to develop an entirely different kind of bomb design (the implosion design) to use the plutonium.

In the end, they had a system that could produce about 30 kg of highly-enriched uranium per month, and about 21 kg of plutonium per month. They also had two bomb designs, one that required about 60 kg of highly-enriched uranium as fuel, and another that required 6 kg of plutonium as fuel. And they had other designs in mind, but that is what they had by July 1945.

So in the end they bet on two methods, and they both worked, though not quite as easily as they thought. Even then, they only had one of each type ready for use in early August 1945, just as a quirk of the schedule (and because they felt the need to test the implosion bomb before use).