The key difference is that Saudi Arabia has not acquired new territory for itself. In the post-colonial era, there are very strong norms against countries annexing parts of each other, with Russia's actions in Ukraine being pretty much the first to happen since the 60s. If the Saudis had initiated the war in Yemen by pointing to some old border dispute and directly annexing the port of Aden, people would be much more angry with them.

For comparison, consider Iraq's attempt to annex Kuwait in the 90s. The world absolutely considered it a five-alarm fire, with the West immediately planning for war and even China and the Soviet Union authorizing a retaliatory blockade. Most people genuinely and strongly feel that the return of territorial conquest as a common practice would be a disaster for the world.