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Don't listen to Tumblrniggers, Tumblr is/was terrible.
You got flooded with unrelated content, the search function was extremely limited and you could only find new things by being force-fed them by some algorithm or other people's reblogs.
Deviantart had the ideal, art-focused setting, with themes, galleries, a (somewhat) decent search engine, etc.
What killed DA is the same thing that killed the entire Internet: phones.
Now that everyone and his mother has access to the Internet, artists want a bigger audience. Facebook, Twitter, Tumblr, Instagram, these are websites for non-artists, or, more importantly, non-creators. Scratch that, it's an entire ecosystem for non-playable characters.
Don't get me wrong, DA was terrible. Like/fave bots diluting actual engagement, hard to report or talk to staff, full of poorly made art, anti-porn policy but still infested by fetish art, etc.
They refused to change with the times and got overshadowed by Tumblr, Instagram and what have you. Had they released their new features (and fixed them) earlier to make a stand, they could have maintained a good chunk of an artistic "resistance" on the Internet. Or they could have Twitterfied the website more and become corporate soulless bullshit, which would have sucked but at least DA would still have a chance in the rat race.
They missed the window by at least 4 years and now it's neither oldschool DA to accommodate the people who refuse to change, nor is it good (bad) enough to compete with Instagram.
The discovery of new art nowadays is completely broken, only favoring bigger artists and hiding unknown artists by default, you have to go out of your way to see anything out of the 3 big names that for some ungodly reason still post over there.