In any market, if someone comes up with a successful trading method and enough others find out about it, eventually, it will stop working.
For example, imagine if a simple 5x10 MA cross worked perfectly on one pair and many thousands of people started using this method. How long would it take until some BIG traders noticed that piles of smaller traders were making some very predictable trades? How soon until they figured out a way to take advantage of it?
On a darker note, how many bucketshop brokers buy up the latest EAs and run them on a fast computer connected directly to their MT4 server? Imagine knowing 1/100 of a second in advance of when a popular EA would open a trade. How easy would it be to widen the spread?