Because we're using it!
We've pretty much got the system down on using rainwater. First, we're fortunate that our tank is located at nearly the highest point on our property, not that there's much slope, but every little bit helps (and you can't gravity feed against gravity...). Nevertheless, we'll be able to completely drain the tank and use it anywhere on the yard.
We currently have four dedicated hoses (going from right to left): (1) for the front of the house, (2) for the back driveway and patio, (3) for the back yard and garden, and (4) to fill watering cans at the tank. Since we're simply using gravity to feed the water, friction losses (loss of pressure and flow due to unnecessary bends, twists, and turns and the roughness of the hose interior) are really noticeable such that even greater pressure head drops across the lot don't fully compensate. Therefore, we've tried to keep the lines as straight as possible to maximize flow.
The coolest thing we've done so far with rainwater is wash the car. Austin is currently under drought restrictions such that you can't wash your car in your driveway, but we (ssshhhhh!!!!) pulled the car into the back patio area and washed it anyway (I'm hoping the restriction specifies the use of city water, but I don't know for sure...). The coolest thing about this coolest thing is that the "waste" water from the washing drips down and waters the grass in the driveway. Sweet!!! No pressure: Just used watering cans to pre-soak and then rinse. This is also the first time we drove a real car on the grasscrete. Held up just fine, thank you.
Watering the grass is a bit of a chore without pressure: You can't hook gravity fed water up to a sprinkler (not enough pressure), so thus far we've been watering by hand. Not a lot of fun because it's been getting so dang hot.
The worse thing about using rainwater? Using the rainwater. Seeing the storage in the tank go down is painful. We have to keep telling ourselves: "It's there to be used. It's supposed to go down." Nevertheless, it still hurts. Which is fine: It ensures we use what we have wisely (the bride wants another 3,000 gallons). And as we enter our traditionally hot and dry months, we're going to need to use as much as we can.