The problem with the Missouri State Guard was that 1) they were poorly trained, poorly provisioned and poorly disciplined 2) their commander Sterling Price was only interested in succoring Missouri for the Confederacy but took little interest prior to Pea Ridge in the other matter of the Trans-Mississippi.
Another problem is that the commander of the Confederate's more powerful force in the Trans-Mississippi (the Army of the West), Benjamin McCulloch, thinks poorly of Price, the Missouri State Guard and think its plans for invasion are unworkable.
Price and McCulloch have such a fued going on that nothing can be accomplished and another General has to be brought in to command them. The choices of that commander were, in this order, Henry Heth, Braxton Bragg and Earl Van Dorn.
Heth turned it down as did Bragg so that left Van Dorn.
Van Dorn was an excellent Cavalry Commander by a terribly poor army commander and his major failure at Pea Ridge when commanding McCulloch and Price's forces in his combined Army of the West gave the Union Missouri, most of Northwestern Arkansas and lead to the redeployment of the Army of the West to the Eastern side of the Mississippi.
Bragg, ironically, would be ideal for commanding this force because he was offensive minded but moderate and methodical. As a battle field commander he was terrible but he excelled in training, discipline and organisation and those were the exact problems the Confederates in the Trans-Mississippi lacked when the Missouri State Guard had most of its sucess.