After some reading:

Legally, the difference is that in the case of a loan the money is no longer the property of the lender. From the moment the borrower receives it, it is his in every respect (on the premise that he assumes the obligation to make a payment for the same amount to the lender at some future date).

So, if the lender were to collect a fee or percentage, he, the lender, would be charging interest for money that was lent, something the Torah forbids.

When there is a HETER ISKA agreement, the money remains the property of the investor (in partnership with the one to whom the money has been entrusted) and the compensation he receives is not “free profit,” but profit that his investment is currently generating