Texas did not join the union because if it did it would be considered a slave state. Northerners did not like slavery while Southern powers believed that it could become part of the other 10 slavery states.
The second reason was that Texas was still territory of Mexico. Texas joining the union would create a crisis of diplomatic proportions and even war.
The Framers chose federalism rather than having a unitary system (all power given to the central government) because people did not trust central government.
