This is because WiFi is not stable enough to maintain a solid connection, to receive both upload and download data. Always opt for a wired Ethernet connection when gaming online, to ensure you will receive the best experience and not suffer from things like latency, high ping etc.
Wired connections offer the best possible gaming experience that is more consistently solid and easier to troubleshoot. It is the better choice for PCs (desktops or laptops) and consoles that have easy access to Ethernet cabling.
