Not enough soil for tree roots. Like a houseplant, once all the soil is used by the roots, the tree can no longer grow.
Poor soil conditions. Anything on a sidewalk can end up in a tree well. Some examples include salt and cleaning chemicals.
Water issues - too much and too little. How can there be too much water? Just like a potted houseplant, if there is no way for the water to escape, the tree can drown.
How is there not enough water? Tree roots travel to get water, sometimes distances over twice their height. Tree roots in wells have limited ability to travel for water and have to rely on whatever quantity reaches them through the grate opening.