It's relatively cheap, and it seems to be supported fairly well (loosely, "good"), so that's two points of the triad. The Law says we can't also have "fast", but that's a reasonable trade-off for those who prioritize "cheap" and "good".
I mean, if you want a build server that runs at Core Duo speeds, you can probably find a used Core Duo workstation or laptop on eBay for less than what a Raspberry Pi costs. While we often talk about how Moore's Law is dead, go back several generations of computers and you'll be impressed at how slow they are. (I'm regularly impressed with how slow current generation computers are!)