>>513339853
It's not that simple. For starters, the early eastern part of the US didn't even have that many natives relative to other parts of the Americas. Moat were also hunter-gatherers. Only those in the lower Mississippi Valley and the Great Lakes region weren't completely. But even so, they really wouldn't be called civilizations compared to their contemporary meso and South American cousins. There also wasn't that much of an opportunity to mix even if one had wanted to in this region. Many of the natives had already been armed with modern weapons thanks to the previous French-British Indian wars. Added to that, some like 5 tribes were allied were allowed to live separately.
The Spanish on the other hand dealt with natives who had been a part of civilization. This incentivized extraction colonialism rather than settler as those natives would be more receptiveto governance. It's simple really, extraction colonialism was more exploitative than the settler one. You're also dealing with unifying a bunch of different native groups with different customs, religions, traditions, fueds, etc into one colony.