Well you see that take over and over because that's what people actually believe in and feel and it's almost weird it has to be repeated over and over. Most protestors are not solidarity protestors. Most protestors show up when they're angry, when they feel like there's something obvious that can be changed and when people around you refuse to do the obvious thing. If you don't have these factors, you don't really get big protests.

For example I don't believe the US saw particularly large scale anti Germany protests surrounding WW2. Before the US joined the war people didn't really know what to do, while after they joined the war there was little disagreement. The Vietnam protests were much larger, because you have the internal conflict and something obvious to do: stop fighting.

People showed up for Gaza protests because they were angry and because they felt people around them, and particularly their governments were complicit in events. People do not show up for Iran because everyone agrees it's terrible but no one really knows what to do, so who are you going to be yelling at on the streets and what would you yell? Additionally events in Iran, relatively speaking probably triggers more hopelessness/confusion than anger, these are not exactly the best emotions to inspire protest

I do protest the massacre, but I don't know what to expect as a result. What do I ask our gov. to do?

Sanction them? To stop sending them weapons? Isolate them diplomatically?

Surprisingly, all is done already