As the article mentions most image hosts simply don't scale to the reddit user base and once an image gets too popular they either pull it or break down under the load. And more importantly many other image hosts add aggressive advertisement and don't allow direct linking. (Imgur recently started to redirect direct image links to the normal image view which has ads for referrals not from reddit though.)

I think the other image hoster which is similar is http://minus.com/. (And min.us, unlike imgur, does not downscale images or force convert PNG to JPEG or increases JPEG compression. Although I think you can prevent imgur from doing this with a "premium account".)

edit: Someone also mentioned https://mediacru.sh/ which is open source.

A lot of that has to do with the fact that other image hosts are slow!