There are a number of answers to this question.

Landfill capacity is finite
Landfills are a finite resource that will eventually run out. Most major cities in Australia are predicted to run out of existing landfill capacity by 2030. As an example, after 2030, waste generated in metropolitan Sydney that's destined for landfill will need to be transported to a site 250km south of the city. You might be wondering "Hey, what's wrong with transporting the waste further? We've got heaps of land!". Put simply, transport forms a major component (almost half) of disposal costs also generates additional emissions. More transport means higher disposal costs and higher emission. Methane Emissions
When organic matter decomposes in landfill, microorganisms consume the waste in the absense of oxygen via a process called "anaerobic digestion". Anaerobic digestion produces methane, a greenhouse gas 20 times more potent than carbon dioxide (yes, you read that right) and while there are continued efforts to capture and utilise this methane via a technology called "landfill gas capture", it isn't always feasible to install this technology and where it is installed, not all methane is capture, meaning a sizeable amount of the methane produced by landfills mostly ends up being released into the atmosphere.