I only started seriously reading Batman regularly at the New 52 so I'm certainty not saying it's always been the case. But at least in that time frame Batman always gets the most coverage. And I'm not saying the quality is always better, I've enjoyed Detective tremendously in the last few years.
I also don't see the same dynamic on the Superman side; Superman and Action have roughly the same level of prestige to me.
Is this something DC has intentionally done? How long has it been this way?