Because "systems" do not work. Never have, never will. But as long as imbeciles are around to buy into it with real $$$ charlatans will sell them
What do you mean by "works"? It was profitable in the 70s-80s before simple trend-following indicators and charts were available to everyone. Back then, you had to have coding knowledge, very expensive/clunky software or plot everything by hand. It stopped being profitable when the markets got choppy in the early 90s and trading software got better and more widely available. I don't have a backtest, but I'm almost sure Turtle has underperformed the S&P 500 (especially when adjusted for risk) since the 1990s. No disrespect to the Turtle creators, though--it was a genius and very profitable system in the beginning. A small group of traders probably made enough to retire in 5-10 years if they managed their funds right. That's a very rare occurrence, despite what the gurus say. And yes, even when Turtle "worked," there were 50% drawdowns that many (most) people couldn't handle. Its still does well during brief periods of market volatility...and occasionally for an entire year like 2008. But it slowly churns your account away the rest of the time. I was on an email list for someone peddling Russell Sands' auto-trading programs. Whenever the market was volatile (like Feb. or Dec. of last year), they'd tell you how much money they'd made in the last few weeks...but never showed long-term performance. There's a reason they do that.
Something similar happened in forex trading around 2004-2006. It was getting popular, and currencies were making big moves at the time. Almost any longer-term system capturing trends/vol worked. I came in too late for that, but I remember interacting with a guy called "Soultrader" on some forums. He started a fund that did great for a couple years and almost brought in enough AUM for the fund managers to be set for life. Then the FX markets settled down and their system fell apart. Same story, different time and trading instrument. I haven't read it, but he wrote a book about the experience called "The Holy Grail Forex Trading System" (and no, he's not trying to pitch the system--just explaining what happened).
We are twice as far removed in time from the turtles as the turtles were from the NYSE being closed on Wednesdays to process paperwork.
You can't selectively choose a few securities and say something still works. ANything can work when one selectively choose the success cases.