Wallaroo started as a company that was building high-performance data processing infrastructure for real-time trading and adjacent systems. Wordpress wouldn't really cut it.

For some startups, that makes sense, yes. For other startups, it may not. See pg's classic essay (that I'm not 100% sure I agree with, but, we're on hacker news) for one example of why you may want to use a more niche langauge: http://www.paulgraham.com/pypar.html

Ruby on Rails was created for Basecamp.

And Rails was extracted from Basecamp. I think startups depend so much on the few firsts individuals that it's hard to have a hard rule about what to use.