Short answer: because the sloganeering has worked.
The supporters of anti-Zionism (which I will define as opposition to the existence of the state of Israel) have managed to capture the public attention with the priorities which they hold dear.
While some people will no doubt argue that this was an opposition to "colonialism" or "imperialism," these arguments are not new. So they cannot be the reason why the treatment of the subject has changed. The change had to come from something new.
"Linguistic Kill Shot"
They have used, largely without push back, a few slogans which serve as linguistic "kill shot," a term first coined by Scott Adams to refer to Trump's unique ability to re-frame political issues, or his opponents' images, by using memorable insults or slogans. The key was not that they were true, or even entirely accurate. The key was that they both imposed new frames and they made those frames memorable.
The gamification of the anti-Zionism campaign has largely developed under the radar.
It's very difficult to argue with a phrase like "Free Palestine" without getting nuanced. Which means it takes less attention to say the phrase than to explain its actual meaning.
And once there was the initial conceptual "buy-in" of the idea that the pro-Palestinian side was on the side of being ethical, the chant "From the River to the Sea" required only a slight cognitive dissonance to be accepted. The same people who opposed violence against civilian non-Israeli Arabs in Palestine had to only "go along" with the idea of some violence being acceptable towards the Israeli civilians. Which is cognitive dissonance for anyone who is neutral on the Israeli-Palestinian conflict, but who opposes violence in general.
While it's tempting to argue that it is the strength of the Arabs' argument that has gotten stronger, I would beg to differ purely on the strength-of-the-argument basis. The arguments are the same as they have been for years. But a larger part of the public is enamored with the slogans.
The slogans short circuit debates. If Arabs' arguments were strong all along, they would have won on the strengths of their arguments many years ago. But they haven't been and it's not because their points haven't been heard. They have been. They are just not persuasive. The gamification of anti-Zionism has been more successful.
P.S. I am not saying that either "Free Palestine" or "From the River to the Sea" are good aspirations, nor that they would survive scrutiny; only that they are very effective in the absence of scrutiny.
Lying With Statistics
In addition to the gamification of normalizing genocidal slogan "Free Palestine," there is also the lying with statistics by referencing irrelevant proportions while talking about proportionality.
The Geneva Conventions require taking steps to minimize civilian casualty relative to combatant casualty ratios. They do not require minimizing opponents' civilian losses relative to one's own civilian losses.
But most people don't notice this fallacy being played on them.
To explain this point, let me label the numbers of casualties as following:
Most meme's trying to prove that Israel's actions are disproportionate talk about high
Which is absolutely wrong.
Victory condition: high
War crime: disproportionately high
Tragical, but not criminal : low
If you combine "victory condition" and "tragical, but not criminal" conditions, you get an uncertain
e_coms/f_coms * e_civs/e_coms / (f_civs/f_coms) = e_civs/f_civs ratio because high * low could be high and could be low, depending on "how high a high" and "how low a low", which are independent of each other.
Talking about a high
e_civs/f_civs, while ignoring a low
e_civs/e_coms, is how you lie with statistics.