Underground parking is expensive and would likely add 1/3 to the cost of the building, plus additional time because they can't start setting the building up there until the ground thaws a bit.
It would take a long time to recoup the cost without charging for parking, plus garages have their own risks. If designed appropriately, they could start charging fees for parking there for games with a shuttle, but I still can't see that working all too well.
To IBs point, if we've waited this long to build an IPF that is just basic, costing $15MM (using Union workers would drive the cost north of those in Fla, AZ), I'd actually be slightly peeved. They need to make a splash with this building.