The reason is Energy Sector has been extremely strong for such a long time that it becomes the strongest support sector to the market. When oil price drop suddenly and the sector shows obvious weakness, market will lose its #1 bullish support. It is not easy for the tons of money deeply rolled into Energy sector to get out in days to enter into other sectors. So the time period before new #1 bullish sector appears, market will be very weak.
Conclution: Oil price drop will not help market in short term. Unless the oil price crashes, market will not get real benifit from that in mid term as well, although some rebounce will appear.