3 to 8 decoders, why are there 3 enable pins?

I still use that 7400 series. The 74138 , 54SN138, and every IC that has *138 in it is a 3-8 decoder that has 3 selection, 8 output, 2 power and 3 control pins. I do not understand is why there are 3 control pins. One is active low, and the others are active high. Why can I not get a 3-8 decoder that has one control line and is in a 14-pin package? Someone made a decision back in 1970, and I just want to know what it was that required 3 control pins. :/