Rate limits.[0]

One can't really build a robust and comprehensive (server-side) search engine on top of Google's search API. In other words, Google Inc isn't going to provide an API so powerful that one can build an "alternative to Google" with it.

On the other hand, if you're talking about a "client-side" filter instead of a new search engine, (e.g. a client-side webscraper that flips through the Google results pages), you'd run into multiple problems. E.g. random CAPTCHAS screwing up the automation script, and the difficulty of creating a "rules engine" (e.g. machine learning) on your local computer that recognizes blog articles with a commercial slant.

[0] https://www.google.com/search?q=google+search+api+rate+limit