For many Chinese, a domain like
a84 is easy to type and remember.
First, consider that typing English words and phrases is very unnatural for Chinese. Even if the user is familiar with typing (and this aptitude would be low compared to developed nations), they would be accustomed to typing single letters and numbers, depending on their IME. Your average Chinese computer user would be typing sentences using keystrokes like
stackexchange would be much harder for them.
Second, English aptitude is fairly low in China compared to most developed nations. Rather than trying to remember how to spell an English word, Chinese would find it easier to remember a short sequence of numbers, and letters if they have to. You can see this in some of the most popular Chinese websites:
Technically we can have Chinese character domain names now, but support is by no means widespread and it takes even more keystrokes to type than simple numbers.
There's also the numerology factor to consider, but
84 is pretty average in that regard (8 is great but 4 is awful).
There's an interesting article on this subject here: http://www.newrepublic.com/article/117608/chinese-number-websites-secret-meaning-urls