Japanese did traditionally use the Chinese logographic symbols for numbers (trust me, I live in Tokyo). However, the drawback of these symbols is that they have no place value. For example, in the number 394, the 3 stands for 3 hundreds, the 9 for 9 tens, and the 4 for 4 ones. However, using the Chinese numerals, one would have to write the symbol for 3, then the symbol for hundreds, then the symbol for 9, then the symbol for tens, then the symbol for 4. The lack of being able to write numbers with the powers of ten being self-evident makes it much more difficult to perform operations like multiplication and division. Just try using Roman numerals for a multiplication problem and you'll see what I mean. What is XXXVII times LXIV? No cheating by converting it to Arabic numerals now!