Search Engine Results Pages (SERPs) are like coffee: we all have a favourite brand we drink every morning and don't really like to change unless somethign significant changes the taste.

People are used to searching with Google and "expect" certain websites in the SERPs and that makesit all the more difficult to "change" and all the more difficult for the other search engines to compete with Google...

At the end of the day everyone sticks to their preferred search engine and will use existing alternatives only if their first choice falls short of their expectations.