I've researched the topic for a while and I could not find a single interpreter discussing the reason (טעם המצווה) for canceling loans after the Shemitah Year.
I know it is a Machloket (see my previous question) whether the loans are canceled or just forbidden to claim, but for the sake of this question, I'll take Rambam's and Shu"A's views that the money is LOST.
"אין שביעית משמטת כספים אלא בסופו לפיכך הלוה את חבירו בשביעית עצמה גובו חובו כל השנה בב"ד וכשתשקע חמה בליל ר"ה של מוצאי שביעית אבד החוב:"
We know it is a DeOraytah Mitzvah
We know that the Torah is extremely stringent on Gezel and money misjudgments, like Ribbit, Honaah etc. Kabbalah adds some frightening details on reincarnations and post-mortem suffering caused by such monetary misbehavior.
A person does a Mitzvah lending his money to his fellow in a sincere hope to receive it back.
Despite the fact that Shemitat Ksafim is linked to returning of the land after Yovel, there's a huge difference - the land is never sold forever, only for a certain amount of years until the Yovel, but a person can lend his money for a week and then 6 years later the debt is miraculously canceled.
The Torah does not differentiate for different types of borrowers - rich or poor - all loans are canceled. They do not turn into a Tzedakkah either. So it's hard to claim a socialistic claim of equality and justice.
I would like to sincerely understand the heavenly reason for justice that is made in this Mitzvah.