Because by definition, they hardly represent the majority of pro-density advocates. If the GP has common cause with YIMBY urbanist environmentalists, then by numbers they are in good company.
The anti local density environmental crowd might represent more dollars, though. That's perhaps the issue.
I suspect, however that the divide is more generational. For the older generation, environmentalism was far more about preserving local wildlands and the wildlife in your backyard. The environmental problems that face the younger generation are far larger in scope and far more deeply intertwined with questions about how we power society.
The anti local density environmental crowd might represent more dollars, though. That's perhaps the issue.
I suspect, however that the divide is more generational. For the older generation, environmentalism was far more about preserving local wildlands and the wildlife in your backyard. The environmental problems that face the younger generation are far larger in scope and far more deeply intertwined with questions about how we power society.