Not my department by a long shot (brown thumb here), but my thought would be poor soil nutrients. Same kinda thing that caused the dust bowl in the US.
Might be time to put in some fresh dirt and reseed. Up here we have to replace our lawns every 8-10 years because the grass basically loses out to the weeds - some combo of weeds being stronger and the harsh winters. At some point you're "lawn" turns completely into cut off dandelions and clover. Basically where we're at right now heh