I agree with misty- if the dentist hasn't told you, ask. Some reasons that come to mind- you're a smoker (they have more problems with implants), you're on meds to strengthen your bones (some cause osteonecrosis, or rot in the jawbone), it's a top tooth and the root is too close to the sinus cavity. That last may apply only to molars; I'm not that familiar with how everything fits together in the head. DH has an upper molar that can't be replaced by an implant because they're afraid they'll accidentally drill into the sinus cavity, which causes something called "dry socket".
You can always get a second opinion, but if your dentist gives you somesolid reasons, anyone who says they can do it anyway should be viewed with caution.