Years ago, the January effect was a winning strategy. Not anymore, after it became well known.
It seems that The January Effect was a real market phenomenon, not a statistical blip or an error in analysis.
It is also clear that although it still occurs and has not been made extinct, the strength of January gains is much reduced, so that a strategy based on the January Effect would be of little value.
The reasons for the loss of potency of the January Effect are the reduction in trade costs, changes in taxation rules (which acted as a catalyst for investors to adjust their portfolios seasonally for tax reasons), and developments of new investment products which are less sensitive to tax regulations and entry/exit costs.
The effect is still in place, especially for small caps, as it wlways was, but it's such a slim margin of advantage it's probably not worth using.
It seems that the market environment changed and this strategy has weakened, which means fewer traders use it. It does not seem that more traders used it so the market actors took advantage of them and made it a loser.