The reason why tumblr become so popular with those types is, that its reblogging system only encourages agreement and creates echo chambers.
You want to argue with someone or add something? Then you have to reblog the entire chain of posts (and wallpaper your own blog with every single reblog) and every follower can see it, while it doesn't get into the public tags. One just doesn't want to shit up one's wall that much.
Yes, they added the comment option, but OPs delete those over a slightest disagreement, this feature seems to offend those people.
Other reason is, that it doesn't cut off edges of image thumbnails, so it's convenient for artsy types.
Personally I find it useless since the time one has to register there to look at pics (protection against harvesting sources for AI, or something), so I can't use that to share photos & short text posts with facebook-less friends, those people ain't gonna register anywhere.