Returning multiple covers is slow. Each cover has to be downloaded before it can be displayed. And you cannot typically download covers in parallel from the same site (as you can from different sites) as that would overload the site's servers. Imagine 5 million calibre users all downloading 10 covers per book from some poor site
For browsing multiple covers, a google image search actually works pretty well. See for example the search the internet calibre plugin. Not as nice as cover flow, obviously, but not too bad either.