They probably needed a rewrite anyway, after learning more about the problem space they weren't happy with the api they'd made thus far.

Go wasn't the only thing that made their new stack better, but you can bet that Go would have made it very difficult for them to achieve performance as bad as ruby gave them.

That said, they are STILL using rails for the frontend stuff and Go for their more computationally intensive stuff IIRC.

They still kept some of their rails code, for things that were easier to keep in rails without hurting performance.