Well if you think about it, Harvest marked the start of a very nasty war. If the BR was in development then, development/research could easily have been pushed aside or halted. Resources would have been pushed to other areas and weapons depots would have been tapped with blueprints of already approved weapons. In order for a weapon to not only finish the prototype phase and get pushed out, materials need to be allocated for the production and also assembly lines need to be built in order for those resources to be put to use.

During a taxing war like this, you wouldn't have all the time in the world to send the shipments (of which could easily be attacked by the more advance Covie forces) of supplies and personel to create such assembly lines in each part of the galaxy where their is a major UNSC outfit.

Just because it's the future doesn't mean everything is all George Jetson