Perhaps you should bring it up to your mother if it bothers you that she isn't showing interested in anyone.  Only she knows the answer to your question (s) with her being single all these years.  Or perhaps, she is content having to be married to one man in her entire life even if he isn't with her now.  

10,000,000 props to your mother for raising seven kids. I'd be fucked up if I had to raise just one!

I think your mother truly loved your father that she could never recover from the shock of losing him. Although I'm generally an asshole around here, I can safely say that whatever your mother and father had was one of those relationships that people get into and are truly happy for the rest of their lives. Losing a lover like that will really knock the winds out of anybody's sails. She may never recover because her state of grief is too severe. That, or she chooses to honor her late husband's memory.

A second thought is that it could have been because your mother was simply far too busy raising kids to go back out into the dating world. A third thought is that although you may want her to date, your siblings may have conflicting views about somebody else other than dad being with mom.

My answer to your question: Simply talk to her about it. If she says she don't want to, don't make her. Remember, that you really don't have a right to make your mother choose whether or not she wants to date or not. Don't make the mistake of raising your voice to her to get a point across though. Mothers, no matter how tall or old we get, can still whoop our asses (dirty fighting. Crude, but effective).

It sounds likely that she just didn't have the time or energy for it before, while raising so many children. But maybe now that everyone is grown up and pretty much handling their own stuff, maybe she can cut loose and have time for it now. I'd casually ask her if she ever thinks about it or wants to, maybe she just has forgotten to think of her own relationship status in all these years and needs a reminder!