I find your question very interesting, because it seems to show exactly why you might not understand how Tecumseh united Indian tribes. I am something of a neophyte in Indian history, but I think I can offer one explanation.

Tecumseh also had a younger brother, a spiritual leader named Tenskwatawa, who was equally important in raising nativist sentiment against the Americans. However, according to historians Gregory E. Dowd, we have generally under-emphasized the role of spiritual factors in shaping nativist rebellion, and instead portrayed men like Tecumseh as isolated, secular warlords.

According to Dowd, there was a general Indian spiritual revival from the mid-1700s, a "Great Indian Awakening" of sorts, which attempted to both re-assert Indian religious traditions at the same time that it promoted inter-tribal unity. Ceremony and millenarianism served as the motor for Indian resistance, creating alliances between diverse groups. Pontiac's War in 1763 was the first large manifestation of spiritual nativist reawakening. It waxed and waned afterwards, with its last great show of force in the War of 1812, where Tecumseh and Tenskwatawa united different peoples but were ultimately defeated by the Americans.

Spiritualism, according to Dowd, allowed leaders like Tecumseh to promote inter-tribal politics. Religious re-awakening simultaneously promoted reclaiming temporal power. According to Dowd, however, historians have preferred secular explanations to religious ones in history. Thus even Indian historical traditions have portrayed military leaders as Tecumseh as "modern" while ignoring the role of spiritual leaders, like Tenskwatawa, because their spiritual nativism might seem "regressive." This misses, unfortunately, how spiritualism allowed for pan-Indian resistance to white colonialism.

Source: Gregory E. Dowd - Spirited Resistance: The North American Indian Struggle for Unity, 1745-1815