Here are some other reasons that occur to me (YMMV):
* A language that is great for performance might not lend itself equally well to experimentation or initial implementation (not saying this is the case for Golang, but in my opinion it probably is for other performance-focused languages like C++).
* You as a developer might have more expertise in a dynamic language.
* The initial development language might have superior OSS libraries which allow you to get to a working application quicker (later you can replace them with your own implementations when you do the rewrite)
* Things change. Golang might have been a terrible choice when Digital Ocean was starting up. Golang is definitely reaching a tipping point as a language, but it might have become a dead-end like many other promising languages and where would you be then?