Because transmission gate literally are two transistors. Standard two input logic gate contains four. So for example you can build two to one multiplexer from t

Because transmission gate literally are two transistors. Standard two input logic gate contains four. So for example you can build two to one multiplexer from two transmission gates for total of four transistors while implementing the same function from standard totem-pole logic would require at least three gates totaling 12 transistors. (In both cases assuming you already have complementary control input, which you usually either have or can share with something. This kind of sharing optimization is the reason why transmission gate usually has complementary input and does not include the inverter internally)