Markets trend in all time frames. In fact, if you make money on a directional trade, by definition, you were in the correct trend for the time you were in the trade. If you lose, you were trading counter trend for the time you were in the trade.
Therefore, the best and most risk free way to make money is to enter a trend early as possible which is just as it is changing, then ride it and then exit when the trend changes again i.e. pick the top and bottom. This is of course impossible to do all the time.
So the trend followers say, well we can't pick the tops and bottoms consistently, so we'll do the next best thing and identify a trend once it has started and then climb aboard, ride it and either exit before the trend ends or just after it ends. So they develop methods of identifying a trend.
The problem is with these methods. If the method chosen fits with the current market reality, then great, that trend follower will make a lot of money. If however it does not match then, the trend follower will struggle.
Unfortunately market realities change. If the trend follower does not adapt his method to the current market reality quickly enough, he is toast. That is why the turtle trend following system doesn't 'work' anymore. It just hasn't adapted to current market reality.