Let's imagine that every rental property goes through a cycle when the owner sees that they are unable to find tenants willing to pay enough money, so they decide to invest money into improving the property, then for some time they think "meh, it's good enough" so it slowly degrades.

When you are looking for a new apartment you are always trying to find the best place that fits your budget, so you will always find it near the peak of the cycle and see it going downhill in front of your eyes.

Just a theory.