Besides the bus contract is with a different company and would've cost the company extra (it was £1 per passenger when National Express operated it many moons ago).
Only reasonable answer I could offer is that Gate 45 is closer to the departure lounge - when passengers have been called for boarding and then dispersed, there is a reluctance to call them again until there is a reasonable assurance that the flight can depart. Calling passengers back to 109 would've taken a fair amount of extra time.