A few things to take into account:
Usually a block of units, (6 pack or more) are larger structures than your average house.
They are also generally taller/higher. As such, accessing the areas that need to be painted, i.e. gutters, fascias, eaves, balcony railings and edges, requires special equipment, such as scaffolding, boom or scissor lifts and cherry pickers. Theses are referred to a EWP (elevated work platforms).
Further to this, items such as gutters, and balcony railings require far more preparation prior to painting which can be labour intensive. Remembering, a average paint project cost can be broken down as 10-15% of the cost towards paint prices and materials and 85-90% towards the cost of labour.