While in general the Chinese are pretty pragmatic with standards and just go with what seems to be the best out there or is at least well established, in this case it really seems to be somewhere in between. The technical reason was that it is compatible to GB2312 which was the pre-unicode encoding for simplified Chinese, so it made smooth transition of existing systems possible, while at the same time being able to encode the full Unicode spectrum. Some services like baidu actually use a mix of gb18030 and utf8 for different resources which I imagine could be rather strange to manage sometimes.