Yes, they should show concern - and perhaps your letter will encourage them to be more sensitive to you and your family. The Bible tells us to "Remember those in prison as if you were their fellow prisoners" (Hebrews 13:3).
Why don't they show more concern? One reason may be because they're afraid they might embarrass your wife or intrude on her privacy by bringing up your situation. This could be especially true if she seldom talks about you, or has otherwise given the impression that she doesn't want to discuss your incarceration.
But she shouldn't feel this way; she needs friends, and she probably needs practical help also. This is a very difficult time for her (just as it is for you) - although it may be hard for people who haven't been in her position to understand this. Pray for her, and ask God to help her find one or two friends in the church who can understand her situation and encourage her. The Bible tells us to "Encourage one another and build each other up" (1 Thessalonians 5:11).
The most important thing I can say to you and your wife, however, is that God still loves you, no matter what your past has been. He loves you so much that He sent His Son into the world to make your forgiveness possible. Open your hearts to Christ, and ask Him to help you rebuild your lives with Him at the center.