This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
If a character was only one byte, you could have no more than 256 characters. The latin alphabet alone uses 24 letters and 10 digits. What about all the other languages with their alphabets and special letters and symbols? There is no way you can represent them all with only one byte.
With two bytes, you can represent 65.536 characters. This seems much more adequate.