I can't answer for the developer, but the answer to that with most small one-person-show projects is familiarity/comfort/ability.
the head-space that adopting a new language for a specific project takes is immense compared to tackling it in a familiar language that you know you're already able in; there is rarely a benefit to doing so outside of team environments where a certain level of on-boarding is expected, or because you have a really niche language requirement/feature that your project is begging for.
the head-space that adopting a new language for a specific project takes is immense compared to tackling it in a familiar language that you know you're already able in; there is rarely a benefit to doing so outside of team environments where a certain level of on-boarding is expected, or because you have a really niche language requirement/feature that your project is begging for.