Why do airlines park their planes out in the middle and ferry passengers to the terminal by bus, even during non-peak hours?
An airport will have a menu of charges for the services it provides to airlines.

Here is one Schedule of Charges for London Heathrow, for some of these services. Probably you can find further examples via their Conditions of use page.

Some of these charges are unavoidable - landing fees, for example - but others will be subject to agreement.

An airline can save money by opting for a cheaper option in certain cases.

If you've ever watched successive plane-loads of Ryanair customers climb out of their 737s into horizontal rain to trudge across 100m-wide puddles to the terminal, and each subsequent shipment of shivering human cargo queue at the steps as the wind whips the standing water up to knee-level, while every other airline during the same period uses a bus to deliver its passengers, you'll have had a very visible indication of they way airlines can choose to pay for particular services - or not.