Moria was a former Shichibukai.
The main forces in OP world are balanced and noone wants to change that, unless being completely ignorant of the consequences (Luffy) so defeating a Shichibukai in Grand Line can only be done by someone powerfull enough and knowing what he was to…
So my guess is that Gecko Moria was defeated by... Dragon ! The only one who has interest in moving the lines of Powers... WDYT ?