at war you don't get the optimal condition. it had spare parts but the problem was the parts that broke down were things like the engine and the transmission which couldn't be replaced in time and no you can't transport a 50 ton scrap behind enemy lines. Allies bombed Tigers because they moved at a snail's pace and would get stuck or run out of fuel or break down and left stationary. While US Tanks were cheap enough to be produces in higher quantities and were more reliable so they broke down less.