It seems to make sense to write a language that maps at a high level to a specific business domain. Query is firmly within the technical domain, not a business domain. It's not surprising that a custom query language would have trouble scaling relative to a commercial query stack from a team of people who live and breath queries for a living and make that living in a commodified market.
On the other hand, there really wasn't anything approaching an off the shelf solution to FogCreek's business problem, and even with massively popular legacy languages there rarely is (it takes an IBM to first productize a COBOL to Java compiler). FogCreek's strategy worked well enough that customer's were writing checks that didn't bounce, and that's pretty good for a software product.
On the other hand, there really wasn't anything approaching an off the shelf solution to FogCreek's business problem, and even with massively popular legacy languages there rarely is (it takes an IBM to first productize a COBOL to Java compiler). FogCreek's strategy worked well enough that customer's were writing checks that didn't bounce, and that's pretty good for a software product.