Normally when visiting a convention or exposition at a large venue or fairgrounds, visitors will have to pay for parking. This is extra revenue for the owner of the building.
Watchtower normally rents the venue including the parking lot. Then the congregations are informed of this, and an amount of donations per pub is suggested to recover the cost of the parking lot.
Watchtower states as a reason that it's much more efficient when not all visitors to the convention have to buy parking tickets, and that's true: at most worldy conventions, visitors don't all arrive and leave at the same time, but spread out over the whole day. Ticket vending machine capacity is fit for that, but not for all JW leaving at the same time. Result: traffic jams which the God of peace and order doesn't like. Hence the inclusion of the parking lot in the building rent agreement (but they may also make some money from this arrangement)
Apparently this year in Canada they either couldn't or wouldn't include the parking lot in the venue rental agreement. Or they did, but they didn't suggest to the publishers a fixed donation amount for the parking.