Because of the way the expansion rate changed over time, as I discussed in my initial comment. The Hubble rate ("how much the universe expands in percent per year") decreased over time. A good analogy is the ant on a rubber rope problem where the rubber rope expands much faster than the speed of the ant, too - but the ant still makes progress on the rope and over time is less affected by its expansion. For the universe, this would be a perfect match in a scenario of constant expansion. That's not what we have, but it's reasonably close for the last 10 billion years.