Ummm, I never think about this before.But as far as I condsider, 

1, it uses alphabetic characters, which in Mandarin in Pinyin but has 4 tones could be the same letters, so the easiest way is to put number after the name...

2,Some people in China dont even use Pinyin/alphabetic characters  but by the process of transforming number of strokesof Chinese characters(bihua 笔画）

3，Chinese are different to Arab language , we cant make up words just by 26 alphabet letters  whatever we want just image you cant think of having a email address in Chinese that easy as your characters may be limited espcially that address should use personal names.

Hope that help a bit...

PS, i have noticed that italki isn't of much use for (for example) children.

If italki practices immediate ban or massive deletion of entries of newcomers. whose behaviour appears to be random, I rather don't like it. It means "failed learning envronment".

Wait a bit... an example appeared in discussions list. But I pressed "refresh" too fast and it disappeared (apparently being deleted)

It was from a chiese student, his skype was given and it went like this:
skott.**** (4 digits)

I have found this post in google, found the student, and this unlucky guy has all the 4 discussion threads of him deleted:/ Now i'm curious, what's going on:/ Is it stalking on the part of italki stuff, or what?

It is somewhat naive to post one's ID within the discussion post. But his other 2 posts are quite typical for here...
http://www.italki.com/user/2630392

Learner SC, yes, I rememner, when I first noticed it in Youtube comments:)
But ultimately, it has something to do with culture, I believe.

Oh, I didn't realise QQ only allows numbers for email address. I never knew anybody with a QQ email until a few months ago. I originally thought it was something to do with Chinese culture ... !