My hunch is that it had a lot to do with who was settling; in a way, it was a little like Kansas 30 years later, except that a lot of Southerners moved there not knowing that they would have a chance to sway which side it would fall on.
If enoutgh Northerners had moved there before, I, too, think they might have been able to avoid it - perhaps with Arkansas Territory getting added into it and two states being created, one free and one slave.
Something was going to be done to try to keep the balance for as long as they could. However, it was only 8 years earlier, before Louisiana was admitted, that there had been 9 years with one more free state than slave state. (1803, Ohio's admission, to Louisiana's in 1812) So, my hunch is that during this time, and even for a few years after, there was acceptance in the South that they didn't *have* to have an equal number.