Is it considered haram not to repay partners their money if I was the primary cause of the project's loss and I pledged to compensate them for the value of the loss, but my financial circumstances did not permit me to do so?

The hand of a partner is a hand of trust over the company's funds, and they are not liable except in cases of transgression or negligence. So, if what occurred on your part was transgression or negligence, then you are liable for the company's funds, and you must pay them to their owners. It is forbidden for you to refuse payment while you are able to do so.

However, if you are in hardship (financially), there is no sin upon you, and they must grant you a respite. And if you are able to work and earn, you are obligated to do so to fulfill your debt.