Programmers make more money because their work is profitable AND demand far outstripped supply. Walmart is also highly profitable but labor supply exceeds demand so low wages.
Google has a profit margin of 20% while Walmart has a profit margin of 2.3%, according to Yahoo! Finance. Labor supply for Walmart probably does exceed demand, but that is only because Walmart uses unskilled labor. Walmart can't pay much more than it does without raising prices, but it also can't raise prices much and maintain its market segment as lowest cost provider.