It's mostly not JavaScript, since it uses the emscripten port of the Tesseract OCR Engine, and if you want to do things in the browser, JavaScript has to be involved.

Ease of deployment. Deploying a client-side JavaScript application remains far, far easier and less expensive than anything that runs server-side (or native compiled) code.

Also privacy: running OCR in someone's browser rather than sending the images back to the server keeps them fully in control of the data they are working with.

Client-side web apps. With today's smartphones, it does make sense to not do everything solely on the server side.

Theretically, cross platform support would be another possibility. But one could argue native C code could be bundled as well, albeit with separate integration being needed. (Android and iOS do support such extensions).

To reduce server side burden/costs

To use it on the web.