Contamination with plutonium-240 was a big problem during the Manhattan Project. They'd done the preliminary calculations based on cyclotron-produced plutonium-239, which was pure enough that a gun design with a long tube was feasible. ("Thin Man")
The plutonium-239 that came out of the Hanford reactor had a much higher contamination rate. This required a much faster assembly of the critical mass in order to avoid a fizzle. The gun design would have to be so long that it couldn't fit on a plane. Whereas implosion could provide the required rapid assembly in a limited space. Thus, "Thin Man" was out, and "Fat Man" was in. Implosion was no longer a secondary option -- it was the only option.
The plutonium-239 that came out of the Hanford reactor had a much higher contamination rate. This required a much faster assembly of the critical mass in order to avoid a fizzle. The gun design would have to be so long that it couldn't fit on a plane. Whereas implosion could provide the required rapid assembly in a limited space. Thus, "Thin Man" was out, and "Fat Man" was in. Implosion was no longer a secondary option -- it was the only option.