Several reasons.For what it's worth, what's interesting is that out of Texas's huge metropolitan areas, only one of them--the Houston metropolitan area--is actually located near the coast:
A.It's large and like California and did have more room for people and major cities.
B.American settlers flooded Texas pre annexation. Like California with the Gold Rush, Texas became a destination for mass settlement(despite for bad reasons, similar to Kansas and Nebraska) and it's population rose. Furthermore the establishment of Texas came at a time when the West whether it be the US Louisiana Purchase or Spanish held California territory had barely been settled. Spain never intended to mass settle the new world they stumbled into ruling the Aztec and Incan Empires. After those empires were taken over, there was really no reason to send lots of people further north, they had inherited the political infrastructure to the south, to the north there was nothing to inherit. Americans flooding to Texas provided infrastructure in the West decades before the Second Homestead Act and the Transcontinental railroad. Up to that point, Kansas and Nebraska the nearest Louisiana states west of the Mississippi had only been nominally settled(barring bleeding Kansas fiasco) there was no reason for large groups to go further west until you had the cities in Texas and California(Salt Lake is an exception but that was a religious migration it's not like there was an infinite amount of people looking to go there. Even today there's really just a belt of major centers in the Southwest(Denver, Vegas, Phoenix, Albuquerque, Oklahoma City)Louisiana and the land ceded by Mexico is largely still empty.
In terms of Mississippi and Alabama those states were(largely)an extension of Georgia in the way Tennessee is an extension of North Carolina. It's hard to say why Tennessee has done(considerably better) and despite being the second main battle ground of the US civil war. But with Mississippi and Alabama I'm going to venture a guess they are victims of New Orleans decline. While New Orleans is still a big city and all In the early 1800s it was THE big city in the South(Texas didn't exist) given it's location on the mouth of the Mississippi. New Orleans didn't have that role anymore with the rise of the railroad and it's hard to imagine Mississippi and Alabama didn't suffer from that.