Because the amount of solar required to power an air conditioner is significantly larger than the air conditioner itself and often you need more solar than you

Because the amount of solar required to power an air conditioner is significantly larger than the air conditioner itself and often you need more solar than you have roof space for buildings 2 stories and greater. (i.e. the total amount of solar you can fit on the roof of a 4 story building will likely not be sufficient to run the air conditioner required to cool that building.)