I think another reason IHT is hated is because it's tax on money you don't necessarily have. Most other taxes are levied at the point of receipt (income tax) or the point of spending cash (stamp, VAT etc.).
For families whose entire wealth is tied up in an illiquid, indivisible asset such as a house, having to sell it to pay the taxman is an emotive and painful event, just as you're dealing with another emotive, painful event.