Many WWE cans are calling for a Randy Orton heel turn. Heck, Randy Orton is calling for a Randy Orton heel turn. Even WWE seems to have realised it is a good idea, if reports are to be believed.
What is stopping them from making the change? WWE is still grooming Sheamus as the top face of the SmackDown brand. Remember that for house shows the brand split is still in effect, even if it is not on television.
WWE do not feel Sheamus has yet achieved the popularity required to be the top drawing face on the brand.
WWE wants Orton to shoulder that responsibility until Sheamus is ready to take the full load himself and is gradually building Sheamus' resume until he is ready.
I disagree with WWE on this however. Their approach has worked brilliantly up to this point. A feud with a heel Orton would allow Sheamus to make the final jump to WWE's elite tier.