Several reasons. There is a seemingly unlimited variation of each basic recipe name (often the same exact recipe named multiple ways; or slightly different recipes named the same). There are thousands of recipe sites and blogs, and ten million unique recipes. The popularity of recipes and recipe sites can change rapidly, and often does; due to seasonality, food trends, seo, competition - so that recipe you found at result #4 on Google, isn't likely to be there a year from now.
Are you sure it's because of the website change instead of the search engine dynamics? The search engine results are not organized, therefore it's unpredictable where a particular results may show up. If the website changes it's keyword combination in tags on the page, it may also disappear from our search results using the same keywords as before.
So we should save it in categories instead of using keywords only. Try the following link and search for "recipe" than click "more similar links..." may give you some idea of how to save an organize your useful information into your private web. (This site is in early beta, there are not much data available yet.)
Any idea why?