Because they signed a contract.

But the contract has a break clause describing the penalties for terminating early for exactly this scenario where you might want to consider leaving (eg. if a better and bigger apartment could be had for $100/week cheaper). I think it's very reasonable for tenants to work out how much the break costs would likely be and then how much they'd save in rent by moving somewhere cheaper, and make a decision on whether it's worthwhile moving. I don't think they have some moral obligation to stay for the original lease term if doesn't make financial sense to them. And I think it's totally reasonable to renegotiate a contract when there has been such a dramatic change in circumstances.

To me it would be similar to someone who holds a fixed rate mortgage with a bank for 3 years at 4% and then seeing all the rates at 2-2.5%, they might want to pay the break fees and refinance with a different bank who can offer them a much better deal. I said it in a comment in another thread recently, but I think people view negotiating with a landlord for a better deal as somehow more personal and even immoral because it's generally an individual investor, compared to having no moral qualms about negotiating hard with banks on their interest rates or utility companies on their bill charges (or even trying to get a better deal with Foxtel by threatening to cancel!) because they're some "evil company," even though you could actually argue that reducing their profit margins will reduce the share price and dividends for "mum and dad investors" (ie. it would have a negative outcome for an individual, we just don't tend to think of it that way).