Not long after the Texan War for Independence, Texas started seeking integration into the United States, most likely in the hopes that they could gain the U.S.'s protection from Mexico. However the administrations between 1836 and 1846 were weary of provoking Mexico. There were some Texan leaders, such as Mirabeau B. Lamar, who believed that Texas should continue as an independent republic. Both of these combined to delay Texas' annexation for a full decade, as throughout that time either one or the other state was uninterested in unification. I don't think the Missouri Compromise was an issue. It was one of the causes for the Texan War for Independence, as the Mexican government was trying to emancipate the slaves. Trying to divide largely unsettled territories between slave and free States was more of an issue as their status as slave or free States had not yet been established. Texas was settled at this time and was not annexed by treaty. Congress simply voted Texas in. That was, and still is, the major challenge to the legality of the Annexation.
