Because like the swastika which went from a cultural Eurasian symbol to what is obviously now a Nazi sign, the “from the river to the sea” is mostly code correlated with antisemitic acts like in the linked article. As you can notice also, Hamas is proudly using the slogan while having called for the complete destruction of Israel in their founding charter while Fatah dropped it altogether.
In 1966, Hafez El-Assad said: “We shall never call for, nor accept peace. We shall only accept war and the restoration of the usurped land. We have resolved to drench this land with our blood, to oust you, aggressors, and throw you into the sea for good.”