Because she's a power whore and she wants some of that "power crackling from his jeans."
Its a marriage made in heaven.
Bill likes his women:
Easily dazzled by power
Liberal
No morals
Easy to manipulate
Plain in the looks department so they are more like to impressed they are being schupted by a former president.
The fact that Belinda is worth millions is just icing on the cake for Bubba.