Which brings up an interesting point to advocate on behalf of the devil.
If they didn't have much control over it then, and Texas became a great state as such.....what's the big need for a fence now?
(donning fire suit)
Because the federal government in Washington, DC abdicated its responsibility for the last four decades with respect to controlling the border. The DemocRATS have championed wide open borders as a way create a dependent population that can be cultivated to vote for DemocRATS. Now that the effects of unrestricted immigration aren't just confined to border states, DemocRATS are losing support from other groups like working class whites and labor union members, so now there is a more bipartisan interest in controlling the border.
There was a faction in Texas that wanted to keep Texas an independent republic. Former President of Texas Mirabeau B. Lamar wanted to expand Texas all the way to the Pacific ocean. He wanted what became the states of California, New Mexico, Arizona, Utah, Colorado, and Wyoming to all be part The Republic of Texas. Of course Texas lacked the population to field an army big enough to defend the area it already claimed much less the area Lamar wanted to annex. Annexation of Texas by the US was the only way Texas could protect itself long term from another invasion by Mexico to reassert claims to the disputed region between the Rio Grande and Nueces rivers or all of Texas.