it's just ViVi. you can also find Namie appearing on the cover quite frequently. among this year issues I've got 1 with Nishino Kana, 2 with Namie Amuro and 1 with ayu. ViVi is so predictable! but I was really glad to see ayu on the Jan cover

it means she's still relevant for ViVi and that's actually very good sign, cause I'm sure this magazine is one of the best selling among girls from 17-25.
and what about others.. well, ayu can't appear on the covers of magazines like
egg,
happie nuts,
ageha,
Ranzuki and else, since they provide only their own magazine's models on the cover. so of course, the amount of the magazines on whose cover ayu can appear reduces.
what about music magazines... hmm, well she doesn't release that much nowadays, so it's understandable she doesn't appear on the covers of music magazines