There's multiple reasons for the lack of unity between the Indian tribes against the European invasion and expansion on their territory. The Indians were divided into multiple groups, some of which had already had problems and bad blood between them, so alliances between tribes like that were not the first choice in mind. Also, the Europeans played well tactically, by promising some of the tribes certain things if they ally with them, so once the Europeans managed to get what they wanted they were turning against their Indian allies who were pretty much isolated bu that point because of their actions. The conquering of new territories by the Europeans also was very big factor, because they managed to cut the connections between the tribes, thus not allowing them to be able to create alliances and be united against them.