It is an interesting exercise to look at the barriers to entry for search engines.
Clearly there are technology challenges (internet scale web crawler, indexer, search interface, ad technology) but these are not novel challenges anymore.
Funding is an issue as operating a general search index is likely very expensive with high upfront investment. Marginal costs are nearly zero as the major expense is likely fixed (crawling and indexing the internet) while variable costs are a small percentage (serving search results for each request).
Google, like Nike, Gucci, and Procter & Gamble, has spent billions to build their brand perception and cement the habit of using their products despite there often being minimal actual differences between competitive products.
But the critical one really does seem to be attracting new users. The history of DDG, ClickZ, Bing all seem to validate this. Just getting a user to _try_ a new search engine is very difficult despite it being "one click away." Certainly a deal with a browser maker / mobile manufacturer would aid this but it's priced amazingly out of the reach of competitors denying perhaps the most obvious way to get trials and traffic.
Google clearly knows this. How can it be true that switching is so easy ("just a click away") as to suggest that being the default is meaningless, and yet pay >$8 billion a year to be that default.