Including the Food Stamp program in the USDA Farm Bill gives the Congressional representatives (Senators and Representatives) from urban areas a vested interest in having a Farm Bill at all.  If the Food Stamp program were not in the Farm Bill, representatives from these areas would not have a reason to support a Farm Bill at all; and there are not enough representatives from rural areas to "go it alone".   Politics?  Yep.  But sometimes that is what needs to be done in politics.