Well I for one choose the last name of Riddle bcuz frankly I love Tom Riddle! and Lord Voldemort!

Well my characters aren't related to Tom Riddle in any way. My charries are from Ghana (I studied abroad there last year and its a wonderful country) and since Ghana was colonized by the British I could say that my charries ancestors were slaves to the Riddle family, and since slaves were given their masters last name thats how they got the last name of Riddle.

And since my charrie is from Ghana I could make her mother the Minister of Magic in Ghana!

It all goes into character planning and development

I think people just fall back on the characters that JK created bcus its easier to use something that works than to come up with something on your own. But once you start RPing you grow in your skill and learn how to mold and shape a character. Or some people just really like a character and want to be related to it or have the same last name as them

but yeah I agree with MuggleBornWitch there are a ton of Weasleys around here