U-235 was very hard to get. They didn't have enough even for a test bomb (the Trinity test already had a plutonium implosion device). Also note that 80% was not the highest enrichment in the device, just the average; most of the uranium was 89% U-235, but other parts used 50%, for an average of 80%. And out of the ~60 kg, only something like 1 kg of the Uranium actually underwent fission, releasing the energy of about 0.6g of mass. The Little boy was an extremely crude nuclear weapon, and nobody expected more than one would ever be constructed.
Bombarding U-238 with neutrons is an easy way to produce the fissile Pu-239; however, this would be of no use in a nuclear weapon. The chain goes from U-238 to U-239, which decays into Np-239 with a half-life of almost 24 minutes. The next step to Pu-239 has a half-life of more than two days. All of this was well known before the weapon was finished, much less detonated. And finally, to get something useful, you would need to separate the small amounts of Pu-239 from the remaining U-238 (etc.); much simpler than separating U-235 from U-238, but still not something you'd do in a nuclear weapon.
The Teller-Ulam ("hydrogen bomb") design is a different story. Much of the energy (up to 50%) in that comes from the U-238 (which also plays double duty as tamper). The fusion serves as a source of a massive amount of neutrons that induce fission in the non-fissile U-238. Again, not an option in the Little boy, though it's likely some amount of U-238 did undergo fission - U-238 cannot sustain a chain reaction, but it does undergo fission when hit with high energy neutrons.