I can give you one of the answers:
They "cant" accept evolution because it invalidates the story in the garden of eden where God creates Adam from dust and then creates Eve from Adams rib. Then they eat a fruit that removes their immortality etc.
This does not work very well with humans evolving from other animals. JWs need to believe that Adam and Eve actually existed becasue Jesus referees to them as actual people (Matt 19:4), so they cant say it is just a story and that Adam and Eve never actually existed.
If they say the Genesis account is fictional or symbolic they would have to preform a lot of other mental gymnastics to explain why Jesus thought they where real. And the JWs don't want to take that route (Many other groups do, but they don't have such a literal interpretation of the bible. JWs are not comfortable picking and choosing as much as other groups, especially when it comes to Jesus), therefore they have to reject evolution no matter what, else they would have to rebuild their faith pretty much from the ground up.
And if they did, they would probably lose most of their members...