Good question! I'll give you 3 of my reasons:
1. In my experience, a majority of people do not kill their spouses. This fact is observable. Of course, spousal killings do occur, but their rarity militates toward presuming innocence on Barry's part.
2. No evidence has been produced -- either direct or circumstantial -- that ties Barry to Suzanne's disappearance. Now, that evidence may exist in investigators' files, but I will not presume Barry guilty on a mere possibility.
3. There's not been an indication of discontent in the marriage. If Suzanne was unhappy in the marriage, is it unreasonable to believe that she would have confided in somebody, be it a sibling, cousin, or friend? Yet, none of these have spoken out to voice such concerns. Even if investigators asked friends and family to remain quiet, it would be highly improbable for all friends and family members to remain silent if they'd heard about worries or fear from Suzanne.