Because the Rankine cycle does not operates between two reservoirs (that is , one of the steps involves heat absorption during a change in temperature). The ideal Brayton cycle, when it operates between two reservoirs does have the same efficiency as the Carnot one, see wikipedia. This cycle sometimes has variations in which it operates across more than one temperature, my guess that is the case with the one described in your book.

Because the Rankine cycle does not operates between two reservoirs (that is , one of the steps involves heat absorption during a change in temperature). Same with the Brayton cycle.