This line from an article on MarketWatch  gives in a nutshell why the stock market continues to rally:

Buoyed by earnings that are proving better than dire forecasts, U.S. stocks on Thursday moved toward solid April gains, with the Standard & Poor’s 500 index poised for one of its strongest months in years. via With earnings bar low, April strong for stocks – MarketWatch.

There is so much pessimism built into the the current economic forecasts that when economies follow their natural cycle and start to recover, the surprises will be towards the upside. The market has continued strong so far in May and the S&P500 is up 35% from the low hit in early March.

With the stock market historically being a 6 to 9 month leading indicator of an economic recovery, if early march was the true low for this economic cycle the overall economy should be showing positive growth near the end of 2009.

More on this topic (What's this?)
S&P500 Earnings Estimates for 2009 and 2010 (Kirk's Market Thoughts, 4/14/09)
S&P 500 Index Returns by Year (Top Foreign Stocks, 4/6/09)
Jeremy Siegel: S&P’s Earnings Wrong (Investment U, 2/26/09)
Read more on S&P 500 (SPX) at Wikinvest