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Several reasons that I can think of:
1. US housing would only look cheap to foreigners. Many Europeans bought property in Florida, for example, they are looking at a decline in value plus a decline relative to their home currency. Unlikely they will be re-investing soon.
2. Unfortunately, no one can predict accurately when the housing crisis ends and the trend is still downward, thus investors naturally don’t want to “catch a falling knife”. Those that will buy will wait for signals such as declining housing inventory.
3. If you are an American investor and the cost of gasoline and the cheap junk we buy from overseas has gone up, then you have less to spend on housing.
On the bright side, there will be a bottom and betting long-term on the US economy is still a good investment.
Perhaps because their value is relative to the ability of the local economy to sustain the market. Those markets more exposed to international influences have a greater influx of foreign investment, likely sustaining price levels or even increasing them. Those less exposed rely upon local jobs. Since we're in a downward cycle, prices in those markets drop.