The easy thing to say is that Tom wasn't aSicilian. But this answer never made sense to me considering Vito's statement that he had made Tom into a Sicilian.

Genco was around during (I believe) several wars between the families, making him a wartime consigliere in that he was more aware of the dangers. Sonny was very angry after Vito was shot and he wanted to get revenge on everyone he could. Tom didn't learn this way, he learned from Vito not to act so hastily. Tom didn't want to kill everyone and start a war between the families like Sonny did. He wanted to avoid a long drawn out war at all costs because he knew it would cost alot of lives and money. Unfortunatly Tom laid back and thought the war was over when it wasnt, leading to Sonny's massacre.