Why did u. s settlers not come to the aid of the fighters under siege at the alamo despite william travis plea for ?

On 24th February 1836 William Barret Travis, the commander of the Texican Rebels sent a desperate plea for his besieged soldiers at Alamo, an old Spanish fortress and mission. He was a Lieutenant Colonel in San Antonio with command of the Texas Regular Troops. 
He sought military reinforcement by dispatching several messages seeking assistance, i.e, "The people of Texas and all Americans in the world" was the most famous address to the people of Texas, and further signed "Victory or Death". Only 35 people went to assist and on 6th March the Mexicans stormed the Alamo where Travis and 190 other soldiers were killed.
Travis defense of the Alamo became symbolic and significant behind the rallying for the Texas revolution whereby April the Texas people and countrymen of Travis fought the Mexicans and gained their independence.