While it would make sense as not everyone has a last name imagine how hard it would be to name your character. A lot of characters have the same name and different last names (and vise versa).
If we'll be allowed to only have first name there will come a time when we will simply run out of options. And if someone takes the name you want - you're screwed, you will either have to change the spelling or add a last name anyway.
And unlike other MMOs we can't use numbers and most symbols.