For me its because most female anime characters are ALWAYS better than their male counterparts or something to that degree. Great designs, cool personalities, enjoyable to watch... the list goes on.
Some key points being...
- They are well designed usually less bland than males, very unique styles. Your typical male character usually has short hair which limits alot of design ideas. Not to mention the males are made fairly bland as to connect with the viewer, the more vague he is the easier he is to connect with in a sense. Females never seem to follow that rule as much, they are always much more unique in their personalities. Also females usually have anywhere from short to long and worn alot of different ways.
- Sex appeal; this can be one as well, alot of female characters can be "sexy" and 9/10 the person behind the avatar is a male and most males like girls. ect~
Meh, I had more but got side tracked and forgot them so whatever~ xD Anyway, think of your top 10 favorite anime characters of all time. I guarantee you that there is at least one female character on there if not majority of the list being female. If you only have males on your list then I would say you clearly haven't watched enough anime or have some sexist issues~ xD My list is actually all females but Alucard, Lelouch and L are all very close to making my list.