Probably cost of training. You'd have to hire consultants to teach best practices and that could cost significantly more than hiring a new engineer. Plus there are other risks to maintaining two codebases in different languages.
* Healthcare startup that basically outsourced its entire infra/security team from a consulting company. The consultants rewrote everything and took over engineering leadership, basically acting as gatekeepers for anything SRE-related.
* Worked at an older DNS company that hired a bunch of consultants to help engineers migrate their web framework from Play to Spring. Said engineers used Play because it was shiny but didn't have the Scala expertise to keep maintaining it, so they moved to pure Java.