Answer:
as simple as possible the Bluebonnet is a symbol of Texas because the bluebonnet ONLY grows in Texas.
Texas has 5 state flowers, all bluebonnets or all species of lupinus, more precisely. It has been used for centuries by native indians, spanish missionaries, painted on cowboy boots and farms, and even a rumor that Texans love the flower so much they passed a law making it illegal to pick them (its not, but don't)
As texas historian Jack Maguire put it "The bluebonnet is to Texas what the shamrock is to Ireland, the cherry blossom to Japan, the lily to France, the rose to England and the tulip to Holland."
Hope thats what you were asking for.
Also it represents beauty