Introduce a foreign object onto the runway and it will inevitably collide with an aircraft. The fire trucks aren't part of the airport traffic management system, their sudden presence is bound to lead to problems eventually.

It wouldn't surprise me in the slightest if the truck has a single radio (airplanes always have two) and was constantly switching between ATC and fire house frequencies. The probably never heard the "stop, stop, stop stop.."

It would also not surprise me if airports previously had dedicated fire services, which have since been outsourced for cost reasons.