#1 Too Much Shade
Grass likes the sun. Between all the tall buildings and our multitude of mature trees, there are very few yards that actually get full day sun. Full day sun is more than 6 hours of direct sunlight. So that little ray you get under the tree coverage in the morning when the sun is rising is not enough. While there is some grass varieties that handle some shade, they tend to be weaker and less able to handle foot traffic.
#2 Foot Traffic
So even though turf is designated for play spaces and welcomes foot traffic more than other groundcovers, the reality is that weak grass and wet soil does not tolerate foot traffic well. So on all those rainy days as people pull up and get out of their cars and run across the parkway, they are inevitably compacting the soil. Aside from that when the soil is soft, it is easier for individual blades of grass to be pulled out with wear and tear. That is why soccer clubs will cancel their games after a hard day of rain even if the current weather is pleasant. All the stop and go action with cleats on consistently rips grass from the soil and gradually destroys the fields beyond repair.
One of the worst culprits for foot traffic on the lawn in wet conditions is dogs. Dogs have to go out regardless of weather so there is no way to avoid them trampling out into the soft wet soil. Typically I see the worst destruction in early spring after the dog has been out all winter and early spring when the snow has melted and the rains have become consistent. Their nails act much like cleats and can wear a bald path into the soil on wet days. However even on dry days dog urine is one of the worst stressors on your grass. Dog urine will burn out the grass and eventually create dead blank spots in the lawn.
Anytime the grass is affected by any of these problems and you have empty soil left, you will inevitably have weeds move in place of the grass. While grass and weeds can coexist to give you a nice green surface, it is not usually the most pleasing appearance for a lawn space. Typically weeds will grow faster and out of scale to the lawn, or just their broadleaf interrupting the texture of the grass is enough to make the grass not look good.
#5 Lack of Water
Besides sun, the other most important thing that grass needs to grow into a lush green plain that everyone dreams of is WATER. While we all want to be conscious of the environment and not water their lawn every day, we have to acknowledge that some supplemental water may be necessary at times. Grass during extreme temperatures, water deprived areas, and of course new sod all need additional water then nature is providing. One of the most obvious water deprived areas are any areas under tree coverage. Trees not only absorb much of the water that reaches the ground, it really blocks a good portion of the rain from even reaching the ground. This is a major contributor as to why grass naturally does not do well in those spaces, so if we are going to force something where it naturally does not do well, it will likely need some additional help, in this case in the form of water. If you need more info on watering guidelines you can find it here.
While I get so many questions expecting there to be some insect or disease to blame poor grass on, most cases boil down to these fundamental issues.