Long history, short explanation:

I adore royalty and I love gold. Duke sounds like a nice rank, far better than earl and not quite as extravagant as prince/princess/king/queen/emperor/empress/god/Pokemon Master(which I hate). DukeGold is a bit too funky, so I added "the" into the set.

The End

P.S. Of course, it wasn't quite that simple...there is a long long history behind it. But that's reserved for another time, preferably never.