We can assumptions on what the reasons were.

Satoshi wanted the supply to be finite, but didn't want to release them all at once, so opted to introduce new ones in blocks. Here now posed the problem that I would assume led to halving;

• If mining reward was left at 50BTC per block, we would have had the entire apply mined within 8 years, after 420,000 blocks.

• This would have caused an issues cause miners will be left to rely on fees was too early and can lead to loss of majority of miners.

• Including halving into the protocol was genius, it eases us into life without mining (coinbase) reward and also factored in how price changes. Early on when the price was low, 50 units should be enough incentive, but as the price increases, as low as 1 unit or less will be valuable enough to motivate miners.