Well, people will obviously compare something to what is considered the best to date... so, they will compare a new SE to Google... cause if you compare it to lets say altavista... well, you look like you are hitting on a fly... not the best sales pitch... 
One thing that makes a difference is often stuff like subdomains dominating searches... bing has a lot of these. It seems to be a fine tuning thing, and the other SE sould just think of a process to clean the first 3 pages properly and THEN, maybe they will sand a chance to take a bite of the market. Cause really... beside webmasters looking for their position in the serps, no visitor really pass the 3rd page.
One thing that has always been, and I doubt it will change, is that power leads to corruption and greed. As long as some are content with what they are getting, all is cool, it's when they wanna pull more juice out of it that the business model changes.
Disclaimer : My posts on SEO are just from my observations and I do not say it is a true fact... A real fact of life is that, I'm often wrong...
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