Limited supply too. "Tin is the smallest of the base metals in terms of production volume, with fewer than 360,000 tonnes produced annually (2011) compared with some 45 million tonnes per year of aluminium. The small size of the market can mean tin faces liquidity issues at times, which can lead to increased volatility and some sharp and extended price moves." Link
Also of importance, currently China is the #1 importer - Indonesia is the largest exporter... All electronics manufacturing uses tin.