Why is the last.fm sharing hidden for deezer family users?

It had been implemented like that a long time ago, so nobody knows why. You can circumvent it when leaving your family and enabling the Last.fm connection in your independent free account, then joining the family again. Scrobbling should still work afterwards, at least that is what a few family members have reported.

Another way would be using external scrobbling apps for Deezer like the Web Scrobbler browser extension or Pano Scrobbler for Android which also has a Windows and Linux version now.