"why the Republican Caucus had to make slavery "The Cause" of the War."
The Republican caucus and the Northern States which followed their guideance was fighting to overthrow the Constitution of the United States and therefore were the ones in rebellion.
They couldn't justify the war on any other grounds politically except to end slavery, because of the Declaration of Independance and the Constitution.
Also after the Emancipation Proclaimation the Northern population became uninterested in fight the war and there was a growing antiwar movement including even being headed by one of Lincolns own generals (McClellan). They were tired of their young men dieing in a war to free the slaves. So the Unionist had to appeal to the Blacks to fill their ranks of the army with USCT regiments to continue the war effort. And Grant used them as cannon fodder to reduce the high numbers of white casualities in his army in 1864 to keep the white northern population pacified, so that Lincoln could win reelection in 1864.
Just my $.02 worth.