For a larger dc motor, 2-pole machines are wasteful of space and heavier. The overall diameter of the motor is set by the armature diameter plus the radial depth of the main poles, plus the required thickness of the case.
Comparing a 2-pole motor with a similarly rated 4-pole motor, although the total field flux is the same, the flux carried by the outer case will be twice as big for the 2-pole meaning that the case must be twice as thick, twice as heavy. In fact it's worse than this, because the main field in the 4-pole can have less radial depth, making the casing bore smaller. The 2-pole machine will also have more wasted space between the main poles.