Question

why did spain colonize and the rio grande valley

Answer 1

Spain colonized Florida and the Rio Grande Valley, if that's what you mean.

Answer 2

Spain wanted to protect its lands north of Mexico City. These lands on the edge of Spain’s claims were called borderlands. The borderlands included parts of what are today northern Mexico and the southern United States, from Florida to California

Answer 3

So, basically, it was protecting itself from other European countries.