Time machine "Don't use" option on disk select
An excellent question. There may be a file on your backup drive called .com.apple.timemachine.donotpresent. Remove it, if it's there, and that should solve your problem. The file is invisible, so you'll have to use Terminal's rm to remove it.

However, with SMB, Time Machine doesn't support it natively, so you may have to run this command in Terminal for the drives to appear. Be sure to restart System Preferences.

defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1 &amp;&amp; killall Finder