Both issues require a change in public opinion.
Nuclear threats are rather abstract at present and basically not preventable in any remotely deterministic way. We could focus on it for a century, only for a hardware failure, software bug, or a simple accident to launch a nuclear missile. That doesn't even take into consideration the power dynamics I mentioned or terrorism. Do we have any clue whatsoever as to how Putin, Jinping, and Trump came to power and stayed in power? Or any clue of terrorism. We don't. If we do in some cases, the cause is not a solvable problem. It's super complex.
So, nuclear threats are abstract, opaque, but yet simultaneously can materialize out of thin air at a moment's notice. However, there are environmental and humanitarian problems that we can start working on and solving today, with actionable solutions.
Environmentalism is a question of balance and sustainability of entire ecosystems and environments. The idea is that by restoring balance, everyone benefits, including humans. How nuclear threats apply to that other than yet another source of environmental pollution or how it’s supposedly on environmentalists is beyond me.
Which leads one to believe that many of these activists are not actually motivated to protect humanity from an environmental disaster, but because they want to improve their social standing.
We're left to assume the worst, that you are not actually motivated to protect humanity from an environmental disaster, you just want to improve your social standing by posting here on HN.
This is in contrast to climate change, which is ongoing and very difficult but not impossible to stop, at least in its worst forms.
In other words, it's a category error. Being worried about climate change doesn't commit one to every way that humanity can extinguish itself (nuclear holocaust, global pandemics, &c.).