The Compromise of 1850 actually undid most of the damage to Southern interests resulting from the Missouri Compromise (of 1820).
The damage was due to the fact that the "compromise" line was 36º 30', the southern boundary of Missouri. That is to say that Missouri was allowed to be a slave state, but all states west of Missouri would be free.
Imagine five tiers of states running south to north including Louisiana, Arkansas, Missouri, Iowa, and Minnesota. The North had three "tiers" of states (excluding Missouri in the middle tier), the South had only two tiers; the northern states would inevitably outnumber the southern.
The reason the South agreed to this in 1820 was because the U.S. had the Louisiana Purchase (the northern tiers) but did not have Texas and California, and the land in between (the southern tiers).
Yes, the South was forced to admit California as a free state, which as it turned out, upset the balance of power in the states. To offset this several major concessions were made to the South.
1) The South quashed the so-called Wilmot Proviso that would have banned slavery in the territories newly acquired from Mexico. This meant that the southernmost tier of states (New Mexico and Arizona) could become slave states.
2) The Federal government assumed the debts of the new state of Texas, after dividing the "old" Texas into Texas and New Mexico, thereby creating the possibility of another slave state (New Mexico). The main reason the South didn't get any further benefits from this was that "Texans" wanted to keep the remainder of the state whole, while other Southerners would have preferred to have it subdivided into at least three or four slave states.
3) The Compromise of 1850 opened the way for a Kansas-Nebraska Act which in theory could have allowed slavery in the Missouri "tier" of states. (They were Kansas, Colorado, Utah, and Nevada, and maybe even California would reconsider.)
4) The South got a version of the Fugitive Slave Law that it wanted.