Google is the dominant search engine within the market today. This explains why web developers and designers tend to have a proclivity toward then. They also provide fairly straightforward documentation, in regards to best practices, through the Webmaster Tools.
Additionally, many search engines tend to emulate the methodology Google utilizes, specifically, keyword placement and saturation, as well as some type of page ranking, dependent upon your link quality (in and out). So, by developing for Google, you're really developing for everyone. It is just simpler to say Google, because everyone knows what you are talking about. To Google is almost synonymous with To Search.