Final answer:
Technical analysis is challenging because it relies on predicting future news and shifts in expectations, which are inherently unpredictable. The concept of a 'random walk with a trend' in stock prices further complicates predictions, despite the general upward trend over time.
Step-by-step explanation:
What makes a technical analyst's ability to predict market movements most challenging is the unpredictable nature of future news and shifts in investor expectations that determine stock prices. Stock prices are significantly influenced by investor expectations about the future, so any change in these expectations can result in a change in stock price. However, these expectations are rooted in future events, which are, by their nature, unpredictable. Consequently, predicting whether the stock price of a company will rise is more about discerning a company that is currently underestimated by analysts but will perform exceptionally well in the future. Furthermore, the concept of a random walk with a trend suggests that while stock prices exhibit random day-to-day fluctuations, they generally show an upward trend over time. This randomness, combined with constant analysis by numerous analysts and investors, makes the task exceedingly challenging.