It is an arbitrary number designed to coincide with the target number of blocks being produced every two weeks assuming an average block time of one per every 10 minutes.

I am not sure if satoshi conducted any kind of research or testing to conclude that 2016 blocks was the ideal difficulty time, however, he did not publish his research/testing if ihe did.

Moving forward, the reason why the difficulty period will not change is that the 2016 block period is the status quo and the status quo is not causing major problems for bitcoin.