Google has spent a ton to increase Chrome's market share - both on adertising, and on bundling it with other products. The Skype installer downloads Chrome and installs its as the default browser—but only if the current default browser is IE. There's a choice to opt out, but the installer's standard action for IE users is to install Chrome as the new default browser.

I imagine the Skype installer deal will change once Microsoft takes over, but Chrome is also bundled in the installers for other popular software like Avast Antivirus and (I think) some of Adobe's products. That sort of deal means a lot of people will get Chrome without actively seeking it out.

Google's annual revenue is 300 times more than Mozilla's annual revenue. Even Google's net profit is 100 times more than Mozilla's entire revenue. And while Google does have many more products than just a browser, their huge cash flow gives them the flexibility to spend for browser marketshare in ways that are just not an option for Mozilla.

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Sources (I also have first-hand reports from friends who've had Chrome installed by Skype):

http://news.idg.no/cw/art.cfm?id=67785482-1A64-67EA-E4A686A8....

https://blog.avast.com/2009/12/03/avast-and-google-chrome/

[Disclosure: I work for Mozilla, but I speak for myself.]