Why was nuclear energy first developed?

Nuclear energy was not developed.  It has existed for as long as time has existed, that is, since the big bang.

The thing that was developed was humans' ability to USE nuclear energy, to do what we want it to do, when we want it to do it.

The reason this was first developed was to bomb the holy beans out of Japan, in order to win World War II.

Today (2020), nine of the world's nations are known to have 14,285 nuclear bombs in storage, for the same general purpose.  Seven of these nations are storing 1,170 of these bombs (about 8 percent), and the USA and Russia have all the rest ... 13,035 nuclear bombs.

All nine of these nations promise that they have no plan to use their bombs, they don't want to use them, it would be wrong and terrible to use them, and they will never be the first to use them, but they need to modernize their bombs so that theirs are better than anybody else's bombs, and they need to keep their bombs for as long as anybody else has any, and then maybe a little longer, just in case.

In the years after the ability to bomb the holy beans out of other people was developed, and enough equipment was built to do it 14 thousand times, the ability to use nuclear energy for other purposes was also developed.  It's used now to generate electrical energy, and to do several jobs in Medical science.