BW and Hawkeye were good at fighting the aliens because they were both elite assassins, they were top-tier military personnel trained and honed to do these very things. Watch how BW handles the aliens in taking them down, disarming them, and using their own weapons on them. All the city officials had going for them were the local beat cops (who say in a talk that the military wouldn't arrive for quite some time.) Quite a big difference there in skill sets.
At the same time the alien "soldiers" were pure cannon fodder, only there for the express purpose of occupying the forces of those they were fighting while the heavier machinery did the bulk of the work for their ultimate goal. So police/military could
have aided but it ultimately wouldn't help and would just be doing exactly what the Chitauri were there for, to die for the progression of their goals.