SPI control for MAX11169
An SPI interface is more complex electronically inside the MCU (digital host) than a simple IO line. A simple IO line that acts as chip select is by its very nature simpler than an SPI input. Hence multiple chip selects driving multiple ADCs that output onto a common/shared SPI input is preferable regards the limited resources an MCU may have.

Of course there is an even simpler mode that you haven't recognized when using multiple MAX11169s - it's called daisy chaning and all ADCs are always enabled and prompted to start a conversion together. The SDO pin of one feeds the SDI pin of the next and the final ADC in the chain feeds its SDO to the host thus, all ADCs simultaneously sample (useful for many applications) and only CLK, SDI and CONVST are needed at the host: -