Because downward trends in rent affect real estate futures, which affects lending rates, which could cause the artificially inflated property values to collapse.
A lot of apartments in my area do first year discounts. The reason for that is a lot of cities have rules about how much you can raise rent in a year. Discounts are a loophole since they can raise it based off the non-discounted value. Also moving sucks and a new place has a chance of being terrible (and you're stuck for a year). So people are more willing to pay an increase once they're already in an apartment they tolerate.