Why is it called pork barrel?

Answer 1

"Pork barrel" came into use as a political term in the post-Civil War era. It comes from the plantation practice of distributing rations of salt pork to slaves from wooden barrels. When used to describe a bill, it implies the legislation is loaded with special projects for Members of Congress to distribute to their constituents back home as an act of largesse, courtesy of the federal taxpayer.

The term is thought to have originated on Southern United States plantations, where slaves were allocated the unwanted remainder of slaughtered pigs, or the "pork barrel." is a derogatory term describing government spending that is intended to benefit constituents of a politician in return for their political support, either in the form of campaign contributions or votes.

Answer 2

Because this is what politicians do -- they lay you over a barrel and pork you.