It doesn't matter if it is the primary cause, it is one of the causes. And it is the one to change that causes the least problems to the people of the country you represent.
In Australia, things are compounded by the incredibly greedy negative gearing rule, which also needs to be banned.
Edit: from an American
That would be when the rent doesn't cover the mortgage interest + maintenance costs.
The benefit is that the dominant form of income in the investment is the capital gains, which is under your control to take at a later date when you'll pay a lower tax rate.
The general strategy is to pump money into a property at a technical loss and make it up the difference on capital gains later.
The negative gearing only applies to ongoing expenses (e.g. maintenance, interest, depreciation) not to capital expenses.
The original idea was that people would lower the rent to make renting more affordable. This rarely ever happens in practice because tweaks to the law mean that you can claim interest and expenses without actually renting the property (set the rental at an unaffordable price point and leave the house vacant).