I had this lens before and thought it was a little better than the Canon version. Its price, I think, was not helping sales because a lot of white box versions of the Canon 24-105 were available at a lower price, and to the casual consumer, they will pick a "big brand" with lots of ads over a name that perhaps only the more advanced users know. Or maybe they just remember Sigma from the old days, before the Art series, and figured it to be a poor brand.
When Sigma introduced this lens I figured it would need to be optically excellent to differentiate itself from the very good and inexpensive Canon L options (24-70/4 and older 24-105/4). According to the reviews, it was only slightly better then the 24-105L and similar optically to the 24-70/4L. Given the low price of the Canon lenses, there was just no reason to buy a brand with a poor reputation historically (prior to the Art series) over a better known and respected brand like Canon.
This appears to be poor market research by Sigma. Their 35 and 50 Art lenses filled a niche in the market that Canon has neglected for awhile. The Canon 35mm and 50mm offerings (other than the excellent 35/2 IS) are showing their age, so the 35 and 50 Art provided photographers with lenses that were significantly better than the Canon L's at a better price point. With the 24-105 they didn't differentiate either in quality or price.