I think it's because professionals and expert users know what good content looks like, which feeds back into the algorithm. The average gardener using the web (

I think it's because professionals and expert users know what good content looks like, which feeds back into the algorithm. The average gardener using the web (not books or other local knowledge) for advice doesn't. Google says "democracy on the web works" which means that what wins is what looks good to the average person among the population who uses that search term.