There cannot be anyone who is able to provide logical reasons for this and all we can do is speculate on what the reasons might be. Given the large number of Bitcoins that remain stagnant in old addresses, the two most logical reasons are:
- The owner of the address no longer has access to the private keys for that address, and this is a very likely hypothesis. That is, the person lost those keys or he died before using that Bitcoin and no one was able to do so after him.
- Unpredictable personal reasons. The most important of which is that the person has enough savings and does not need to exchange that old Bitcoin.