Call me cynical, but I think this is because it is a very simple way to set up the main character as a solitary and vaguely tragic figure. Especially in the case of a man as the protagonist (which is mostly the case, since women as main characters are reasonably few and far between) this then becomes quite a useful situation to get him/her to emote and respond to the various incidental characters that will, no doubt, appear on the way. As well as the fact that they also have no family/spouse/mate to lean on at trying times, which makes it more believable for them to turn to the screen buddy/colleagues etc. for help and support.

You very rarely see the main protagonist with a normal and supportive family unit behind him/her. Even when canon provides that, the link is sometimes removed.

Look at Daniel in SG-1, where the first that happens in the TV series is the removal of his wife. After that, our hero can be more or less written as a free agent and any support system provided by a selection of other important characters around him.

I know this is a very simplistic analysis but simple works for me.