If the Civil War hadn't intervened would Congress likely have passed this act? Why wasn't the idea revived after the war?
Well, the problem was quite complex thanks to the Mexican War and the Bear Republic revolt in Sonoma led by John C Fremont. At that time, Alta California was a province of Mexico and the revolt was quickly resolved - mainly by Gen Maria Vallejo, who favored the province being annexed to the United States. (He came to really, really regret that position but it seemed like a good deal at the time!)
Don Pio Pico was the last Mexican governor of Alta California, whose capital was Los Angeles. Most of the Californios had their ranchos and haciendas in SoCal - with the end of the Mexican War, Alta California was ceded to the United States and shortly thereafter became a state. Before the province was ceded, Pio Pico had tried very hard to get a foreign nation like Britain or France to take possession but that was squelched by the Polk administration. In 1850, California entered the Union as a free state, which tipped the balancing act of free/slave states. Some have argued California started the CW! At any rate, now Don Pio Pico became a state legislator, and introduced an act separating SoCal starting at San Luis Obispo, renaming it the territory of Colorado - and this was passed by the state but not voted on by the people.
After the war, slavery no longer an issue, having had the gold rush and lots of agricultural colonies started by Southern refugees, it was never thought of at a national level to split the state but instead to build a railroad to get to it easier. So...the Pico Act died without being enacted.