Because otherwise it could be "mummy" for the little ones and a mummy is not something a mother would like to be called, methinks. XD

I know! xD I came across that term in British English.

Sure, we learned that term at school, too :)

But I do use mum a lot, too. :3

Always use Mum and mummy when I was younger XD

Mums for the win.

Lol why would a mother not want to be called "mummy"?

Mummy. Like the things the Egyptians made their pharaos into when they were buried. ;)

Haha now that I think of it they're sorta pronounced differently...

"Mummy" has more affectionate implications, and it's pronounced more...affectionately? Lol. Hard to explain.

Mommy but close enough.