Shulchan Aruch, Choshen Mishpat 292:7 states the following halachah (from Bava Metzia 43a):
If Reuven deposits money with Shimon (a moneychanger or storekeeper) for safekeeping, Shimon is entitled to use the money for his business and replace it with other funds. (This is different than other kinds of deposits, where he is expected to return the same item as was deposited with him.) Once he actually uses the money, he is fully responsible for anything that happens - even if it was lost in an unavoidable accident - and will have to repay Reuven.
Now, presumably, anytime Reuven deposits something with Shimon, he can stipulate that Shimon should rent the thing out and they'll split the profits.
So now, take the case of an iska (a partnership where one person puts up the capital for a business, the other one will manage it, and they'll split the profits). The Gemara (Bava Metzia 104b) characterizes such an arrangement as "half loan and half deposit," and therefore this creates a problem of ribbis, because the managing partner is effectively doing the financier a favor in accepting and managing the deposit in return for having received the loan. In turn, this necessitates a heter iska to make the transaction permissible (by stipulating that the managing partner will receive a wage for his labor, plus other details intended to reduce the financier's exposure to risk as much as possible within the parameters of the laws of ribbis).
Why, though, can't we just do it more simply? Why doesn't halachah just declare all of the capital a deposit rather than a loan, and then there is no problem: the managing partner is fully responsible (or, for that matter, he and the financier can arrange other terms, such as that he's responsible for only half of the money), and they can split the profits freely without any ribbis problems, because there's no loan?