power - Why does it take so long to make a nuclear bomb?

So as I know nuclear bombs are derived from fission reactions: By providing the nucleus with enough power to trigger a chain reaction. If uranium was present why does it take so much to make a nuclear bomb.

Answer

If you throw a bunch a uranium ore in one blob, nothing happens.

If you chemically purify the ore so that the only element present is uranium, still nothing happens.

The runaway chain reaction needed for a uranium-powered bomb involves U-235, an isotope having three fewer neutrons than the most common natural isotope U-238. According to Wikipedia,

The required material must be 85% or more of ${}^{235}\mathrm{U}$ and is known as weapons grade uranium...

Now U-235 and U-238 have essentially indistinguishable chemistry.¹ How do you purify the material to be mostly U-235? The traditional technique is to make the gaseous compound uranium hexafluoride, $\mathrm{UF}_6$, and use some mass-based physical process to separate out the ${}^{235}\mathrm{UF}_6$ from the ${}^{238}\mathrm{UF}_6$.² This is slow, tedious work. In fact, the Oak Ridge facility built to do this during WWII was by some measures the largest building in the world at the time. It extracted U-235 one tiny bit at a time, and even after months of operation, there was just enough material for a single bomb, with nothing left to spare to even test the device.

For plutonium weapons, there is a similar complication. Bombs are made from Pu-239. Pu-240 is too unstable, and if there is too much of it in your bomb, it leads to a premature reaction (by a small fraction of a second), scattering most of the fuel rather than detonating it. The problem is even more difficult given that plutonium isn't found naturally in large quantities - it is produced entirely as a byproduct in uranium-based reactors.

In the end, it takes a large industrial infrastructure to manufacture either type of fission bomb since you need large amounts of an isotopically pure substance.

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¹ Even the chemical difference between normal hydrogen, consisting of 1 proton, and deuterium, which is a proton and a neutron, is small. Changing the mass of the nucleus, even by a factor of 2, does very little. Now imagine changing the mass by a factor of only about 1%.

² Note that there is only one natural isotope of fluorine, F-19, so the only difference in masses for the molecules comes from the uranium.