Another factor to consider: convenience. It's easier to deal with the extended family by having them dead (unless family members can contribute to shows) than having the characters reference them as often as 'normal' people do, or by making the main characters seem strange by almost never referring to their extended family over the course of seven years.
Interesting thought. TNG kept the crew mostly far from home, while DS9 was more rooted and maybe had more opportunities for family-related stories.
On the other hand, though, TNG consciously incorporated the idea of family into its premise, with the ship designed to accommodate crew families; and yet it didn't take advantage of this with any of the main characters besides the Crushers. Also, VGR kept its crew much further from home, yet explored its characters' families more than TNG did.