The first, and biggest, reason is that humans will soon master the art of manipulating DNA. This will stop evolution right away because people will only change due to their will, rather than adapting to nature through trial and error. This brings me to the second reason for this.
Human beings have mastered the ability to alter their environment to suit themselves. In addition, human beings have separated themselves into classes of individuals, totally screwing up mating patterns. Thus, we don't have survival of the fittest anymore, nor do we have females mating with the strongest men either (or the other way around). This puts a wrench into the gears of evolution, since the driving forces of evolution have been cut off.
So while I say evolution is over, we aren't stuck. I am pretty sure scientists will begin manipulating DNA in unborn children wuthin the next 10 years, and within 50 we will be able to "design" our children (although legally, people will be limited as to what features they can give their kids).
A very scary proposition...
Yessirree it's the same old 'Man Conquers Nature' story with a new headline:
"DDT Ends Crop Failures"
"Penicillin Ends Bacterial Infections"
"Cloud Seeding Ends Droughts"
And now, ladies and gentlemen, in the far ring, before your very eyes, Captain Science
"Ends Death and Expands Intelligence Infinitely with Genetic Manipulation".
No longer will mankind be subject to the tyrannies of nature that wash mountain ranges into the sea, cause stars to go super nova and galaxies to wheel one into the other. Thanks to mans superior intellect the stupid cruelty of nature will no longer be able to compete. Step right up folks, tell your grandchildren that you were there when Captain Science pinned God to the mat.