Why use IEEE 802.15.4 over WiFi?
It's low powered for sure and can easily be battery powered. It can support up to 64,000 nodes and if you are data collecting small amounts of data from multiple locations this is a benefit. Transmission bandwidth is smaller than WiFi which means the receivers are more sensitive and, for a given power transmitted, the "error free" transmission range is bigger.

When multiple devices are used the nodes can form together and act as repeaters for more distant devices thus keeping power low. The actual zigbee radio (node) is just a chip plus an antenna i.e. it's relatively small compared to a typical WiFi solution.