Broadly speaking, that is because we're approaching something like a carrying capacity, estimated to be somewhere around 10B.

If we increase land use efficiency dramatically, that effectively raises the carrying capacity and population growth can continue much longer

The population growth decline is not tied to the carrying capacity but to the development of the economy. Countries that are further economically developed have lower birth rates and vice versa. For example Finland is mostly a country of deserted forest, yet it has a very slow growth rate. The country could carry a population of 50 million easily, but is hardly growing anymore at 5 million.