There were many reason the US broke all the treaties with the Indians.  Sometimes it was a treaty made by people who left the government, sometimes it was a loop-hole created by misunderstandings of the Indian people concerning what the treaties said.  Basically, they didn't work because of the greed for land and expansion.

Due to the Western society's longstanding effort to exterminate indigenous cultures in much of North America, the theft of Indian national territories was not carried out to open the land for white settlement, but rather the other way around. Native lands were coveted by railroad, mineral, logging, and other interests, which widely advertised the promise of free land. Settlers were sent to claim land within the sovereign territories of many Indian nations, often unaware of or ill-prepared for the hostile reception they would face. The inevitable clash justified a “rescue” by federal armed forces, thus securing the land for business interests. Tribal leaders were convinced, coerced, or tricked into signing a total of 371 treaties up through the 1870's, ceding almost all their land to the government, save for some small reservations. None of these treaties was completely honored by the US Government.