For me, two things come to mind.

First, the previous pacifist generation got older and started finding excuses advocating for war. What used to be a no-go in the left-leaning crowd somehow has become a mandate, because everyone else is fascist and needs to be stopped or something.

An example I am very familiar with from when I was growing up is the Green party in Germany.
Initially they were consisting of pacifists, environmentalists and openly non-conformist people. After ending up in power some time later, their votes changed legislation so the German army could operate outside its own borders, a restriction put in place after WWII.

From there it only got worse over time. This is them now:

But no party has changed more than the Greens. Founded in 1980 as an anti-war party, they have in recent years become fervent and bellicose advocates of rearmament. Anton Hofreiter, who for many years co-chaired the party’s parliamentary group in the Bundestag, is continually demanding ‘arms, arms and more arms’, and criticises the SPD’s refusal to send Ukraine Taurus long-range cruise missiles.

(source)

So that leaves the old "OG" pacifists out of the picture as they've become too complacent or ideologically blinded to see war for what it is.

The second part is that there is no new generation taking up the mantle.
In part at least I suspect it's because expressing discontent has been turned into this frictionless experience.
People think attending some half-assed rally once and signing a petition means they've done their bit and they retreat back home in the expectation that someone is bound to listen. Or they'll start some social media challenge to "bring attention" to some cause. While only they themselves and their close circle even see it.

And when inevitably no one listens, it's easy enough to distract yourself in other ways and blame the older generation (right now: boomers) for everything.

And no, I'm not dunking on the current young generations. My generation didn't do shit either. We were for the most part apathetic and/or nihilistic.
We had a few people who'd stick their necks out, but no where near the magnitude of those in the 60s/70s.
We're just lacking a generation pissed off enough and not too distracted to notice they're being abused. And have been for a while.