It's a fairly simple thing. A-listers don't want to give out much personal info about themselves it's really not that good for tabs, or the fairly new phenom of chav-mags, most of these mags didn't exist 15 years ago. Now Closer, ReVeal, Now, New, Star, Love It, need people that will give them everything to fill up the pages.
So the ones that sell everything will be giving top-billing, whilst stars, even the lower list ones that don't tell all to the press will be ignored. Ironically they'll say of these people that don't sell that they're fame-hungry and worse, when they do something, because in these mags eyes people who don't sell everything betray them.
Create stories for them, and they'll fawn over you when you make your latest comeback. Also it's interesting that in general it's the women that get the front pages, so they'll spin it better for them. After all the sex dungeon thing about Alex Reid has been proved to be hyperbole, yet it still is repeated.