It began as a venture by a couple of graphic artists, focused on artists. It happened to have (for a few iterations) a quite clean design based on Markdown, and (with a couple of what should have been reasonably small tweaks) could have done well in the long-form text+images space (now largely owned / failed by old-school blogging engines such as Wordpress, Medium, and Substack).
The features it had, including Markdown's sections and tables was why I'd posted that particular take on G+ there. It simply presented the information better than any other readily-available platform I was using at the time.
In 2015 a bunch of trans activists and sex performers who'd been hounded off Facebook by its Real Names policy wound up on Ello, and a few people arrived from other platforms (I was at the time a refugee from G+). For about six to twelve months it was the New Hawtness, and attempted a few pivots to the space, but performed fairly poorly in doing so. My own Ello profile has a link to a set of curated posts, one section of which details various gripes and failings.
One thing Ello did do was organise as a B-Corp with a community focus. That went swimmingly until it didn't, with the first accepting VC money then being sold to a collector of such sites. The B-Corp language seems to have disappeared, along with the community principles texts. What's left is now ... a sort of vaguely art-aggregation site with an interesting history but little future so far as I can tell. It's burned virtually all its early adoptors (myself included).
That said, Google's stated communications regarding Google+ had and have been questionable from the start. I'd had my own part in this in addressing the true size of the active community on the site, which was far below the 3--4 billion listed profiles and many hundreds of millions of active users touted. In practice, probably closer to 4--6 million true frequently actives within 30 days or so (itself not unsubstantial), and perhaps 100 million who'd been active at some point.
<https://ello.co/dredmorbius/post/naya9wqdemiovuvwvoyquq>
But I'd take that stated reason with a large dose of salt.