Because when Shias portray the Prophet Muhammad and his family, they are beautifully painted works of art, made with respect.
Our artists do not produce pictures of hook-nosed men with bombs strapped to their chests (or wrapped in their turbans), or drawings of of pigs and dogs wearing turbans. There’s a fundamental difference there - I’m surprised you’re having trouble understanding it.
That said - I think the response to the cartoons in the Muslim world was completely inappropriate. Boycott their goods as much as you want, but violence is completely uncalled for.