DeShields' first pro season has been a mammoth struggle. The eighth overall pick in 2010 has slumped to a .220/.301/.308 batting line with low Class A Lexington after going 1-for-20 with eight whiffs this week.The above quote is from Baseball America. Delino DeShields plays second base for the Astros.
Here's a second reason. According to High School Baseball Web, "only 5 to 6 percent of drafted players ever play a day in the major leagues."
Finally, an article in Baseball Prospectus states that in 31 drafts since 1978, 51% of the players drafted in the first two rounds have made it to the major leagues. That means that almost half of those draftees didn't make it to the majors.