why there is no quad 1-to-4 demultiplexer?
In the days when it made sense to put such simple logic on its own chip, the standard package was the 0.1inch pin spacing DIP. Standard packages were 14 and 16 pins, later 20 pin packages were added; anything larger than this was an expensive oddball. 

So count up the pins you'd need for this circuit:


16 inputs
4 outputs
2 selects
probably an enable or output tristate (G) pin for expansion
2 power supply pins
Probably 25 but an absolute minimum of 24 pins.


So you're looking at an expensive 28-pin package and a vanishingly small market compared to the commodity parts, which do the same job in two 16-pin parts. 

Simplify your inventory, reduce costs and live with the tiny extra board space.

When PALs and later FPGAs came along, you could roll your own (with no Enable!) and the same 24-pin part (probably a 22V10) could fill several roles on the same PCB, so you could buy in enough quantity to make its high price acceptable.