Why do 74 series IC's have two ENABLE pins
It's just simply to reduce the "glue logic" needed to implement the device.  It's simple enough to just tie the one input to low and to use the other pin as a single input and this gives more flexibility in implementation.

If you look at the package you realize that without the extra /enable that package would have an NC pin.  I suspect the first designers decided that having a little extra functionality for "free" (i.e. in pin count) would be an enhancement.