a. She's the lead vocalist
b. She has more camera time (check all the PVs)
c. Like what Eugene said, she has this extra factor that really makes her stand out.
As for the other things, I'll have to disseminate them equally:
HARU - same reasons above, plus she handles a crowd well. A sign of a good leader. As for interviews, I find myself always looking forward to her speaking, I don't know why, but I guess its because I'm used to it. HARU, for me, represents them as a group, but the idea of them being one is not lost.
TOMO - With a huge bass, dancing, singing and keeping a great image (THE SMILE)..I give kudos to her. It's not everyday you see a bassist that's really good, and at the same time able to do such. I see TOMO as the most versatile person in the group. Though, she has this slight awkwardness that is truthfully adorable. Seeing her different and serious side in Shunkan Sentimental, I'd say her fans will always be entertained.
MAMI - Like what others have said before, she has this cool and calm aura while playing live that makes her appealing. She doesn't do much, but concentrate on playing, dance a little, sing but never would stand out. Though I digress, she doesn't lack anything from doing those things. It actually makes her look better.
RINA - Being at the back at most lives and the PVs, it gets hard to view her, especially, since the other three would have more screen time. But she does look great in most PVs and lives, always smiling and keeping her head high, now that's a great drummer. In their interviews, she really shines. I never really thought of RINA as a bubbly person till I saw some of their interviews. Stoic as she may seem, RINA's got a lot to show for us fans.