That's because they realized it's actually a pretty crappy social network.
But that's just the start of it.
Its terms of service have nearly all the content restrictions Twitter does, and some, like its prohibitions on vulgar language, are stricter. And they reserve the right to take down anything with no explanation.
You can't even unlock all of its features, like direct messaging, without submitting your Social Security Number and a photo of your driver's license, and they don't give any compelling reason why.
There's also a forced arbitration and indemnification clause. You can't sue them, and if *they're* sued, they can hold you liable for their damages if they decide your content is at fault.
That inherently cripples it as a useful social network because it has no reach on other sites.
Already most politicians who jumped on it are abandoning it.