SpaceX can't grow money on trees, of course the BFR has various commercial uses. The commercial demand for flights to Mars is pretty low, and if they built a vehicle that only makes sense for mars they would have no money left for any investment in Mars (sending colony ships or whatever).
But for getting the most profit out of LEO launches and suborbital passenger trips they could have built a much smaller rocket, slightly bigger than the Falcon Heavy. As $/kg for launches goes down, you expect average payload size to go down, not up (since it makes less sense to pay for overengineered satellites with lots of redundancies).
ha - the opposite, more like! "Overengineering" is what allows payloads to be as light as they are. You spend a fortune squeezing every last gram out of the design because it costs so much to get it up there. Lower launch costs allow heavier (cheaper) stuff to be economical.
But for getting the most profit out of LEO launches and suborbital passenger trips they could have built a much smaller rocket, slightly bigger than the Falcon Heavy. As $/kg for launches goes down, you expect average payload size to go down, not up (since it makes less sense to pay for overengineered satellites with lots of redundancies).