i don't think that its her being more popular than the other girls.
i feel that Lisa shares more of her personal life with us.she has 2 young boys and a baby on the way.its just that people relate to her more as a part of there own family.i bet every single person on this forum is just waiting for the baby to arrive and will be worried until then.
the other girls seem more private with there personal lives.
Hayley and Chloe have given interviews and shared their lives with us in many many interviews over the past few years. I would also add that Hayley is probably the most popular of all the ladies if you count CD sales and concerts.