Most guides and environments are geared towards Ubuntu, since it's considered to be more noob/user-friendly. It's possible to setup the Android SDK and a build environment on Fedora, Scientific Linux, and other Red Hat derivatives (iirc, CentOS doesn't have some new-enough packages, but I haven't tested CentOS 6.0) but it's considered more difficult. "True" developers (bearded warriors) don't give a damn either way because they build every package from source, and solely use the console and 20 screen sessions.
tl;dr, it's just easier to setup and get running, and most developers either don't care, or like Ubuntu's shinier graphics.
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