Please allow me to answer your question.
It is fairly simple to answer because it revolves around the TREND of the market.
When a market is in a strong downtrend, it tends to stay in OVERSOLD territory. Take a look at any Bear Market or Bear Trend and you will see factual evidence supporting this observation. Lower lows and lower highs equal a downtrend, which equals an OVERSOLD condition that stays oversold.
On the otherhand, when a market is in a strong uptrend, it tends to stay in OVERBOUGHT territory because higher highs are being made, as well as higher lows.
In otherwords, overbought/oversold conditions are reflective of the actual TREND.
Pretty simple.