Why do people use bigger cassettes for lower gearing when they could instead use smaller chainrings?
Gear ratio range.

If you decrease the chainring sizes you decrease the highest ratios available. It's not possible to make the gap between the chainrings much bigger and get decent front shifting so the large ring has to shrink with the small one.   

It's easier for manufacturers to make a wide ratio cassette that retains an 11 tooth sprocket and shifts well.