Many browsers support some formats that are favorites of some vendor, but the new formats are not supported cross-browser, so the benefits of switching formats are reduced and the costs are raised -- in particular, one benefit of smaller files is reduced storage cost. If you have to support JPEG for legacy and then something else for Microsoft's format and something else for Google you are increasing your storage cost, encoding complexity, etc.

I looked at alternative image formats for a photo site I was working on and never convinced myself that the benefits of switching formats was worth the trouble.