Because the major barrier I see is, with the climate change problem in general, no one is able to concretely and incrementally connect someone's (tiny) polluting action to actual harm to someone far down the line. (for legal liability purposes -- not saying the link doesn't exist in scientific proof)
Let me phrase it this way. For someone to have an actionable legal cause, doesn't it require that the person/entity causing some claimed harm have a particularized, concrete, connected relationship to the harm? And that their cessation of the claimed causative action would remedy the issue? this is the important thing
The kids claim that climate detrimental action is causing them harm. In most cases (some gas guzzling vehicle, coal burning plant) etc. the thing they claim is harming them, even if it were to be completely shut down, would not solve the climate problem. They would still be experiencing the harm.
So how would a law be enforceable if anyone could be sued for something that minutely adds to climate change, and their stopping their activity would produce no measurable effect on the claimant's outcome? Heck, the kids' own existence could be said to be linked to climate change.
Or, how about this -- wind farms could be sued for minutely adding to the environment / climate change problem. Or big box retail stores for causing traffic and wasteful packaging. The list could go on and on. We have opened a can of worms like if you said that denial of "the pursuit of happiness" is something that people can sue over.
What legal principle is being promulgated here? I suspect this will be subject to significant review if more cases accumulate.