The introduction to the font says the following:
The Android design language relies on traditional typographic tools
such as scale, space, rhythm, and alignment with an underlying grid.
(...) is essential to help users
quickly understand a screen of information. To support such use of
typography, Ice Cream Sandwich introduced a new type family named
Roboto, created specifically for the requirements of UI and
(Emphasis is mine)
Another page states:
While some grotesks distort their letterforms to force a rigid rhythm,
Roboto doesn’t compromise, allowing letters to be settled into their
natural width. This makes for a more natural reading rhythm more
commonly found in humanist and serif types.
So you could easily conclude that google simply wasn't satisfied with how other fonts looked on high-resolution screens (especially small screens).
Since more and more content is consumed with tablets and smartphones, it's also reasonable to not limit the usage to apps, but also use it on webpages and other digital media.
Another thing could be possible license issues.
In the end I can also imagine that google just wanted to create something new and fancy.