If it was a poly the voices would be far less powerful, the IO would be lacking, the chassis would be rudimentary and the price would be much higher.
Ref. DSI as an example:
Pro 2 @ $2200 => 4 oscs, 4 lfos, 5 egs, 2 filters etc.
OB-6 @ $3300 => 2 oscs, 1 lfo(!), 2 egs, 1 filter etc.
May I also offer a better car analogy:
A poly is like a people carrier - lots of seats, dull and underpowered.
A mono is like a sports car - few seats, exciting and powerful.
Edit: hah, the dangers of generalizing - the PB is not dull and underpowered, although a bit less dangerous than the MxB