Mainly, because sometimes we encounter observations that appear utterly inconsistent with the big bang model of the universe. But if the cosmological constant is included in Einstein's equations, the solutions allow a much greater variety of possible models for the cosmic expansion than the Friedmann solutions. Just a few years ago, it seemed that there was a fundamental contradiction between different measures of the age of the universe. Moreover, the best estimate of the ages of the globular clusters was about 15 billion years. That would be a scientific crisis.
Cosmologists are loath to include a cosmological constant in their equations.
They would like the universe to be simple. But if forced to choose between including a cosmological constant or giving up the theory of the expanding universe, they'll include it. Now, two recent observations have forced us to bring the cosmological constant back into the picture. The only way to have an accelerating universe today is to have some kind of long-range repulsion in the universe that overcomes gravity. That's exactly what a cosmological constant does.