HTML file uploading doesn't work. Live logging involves listening for changes to a file and only sending up the new stuff. The uploader also creates a new format that is less than 1% of the size of the original file, which is critical for making the uploading fast and conserving bandwidth.
Adobe AIR obviously would not be my first choice for a client now, but back in 2013 when the site was first being created, there weren't very many good alternatives. It was basically Qt, Silverlight, Java or AIR, and AIR was the best of those for doing the interface in HTML/JS/CSS. At the time (2013) it supported Linux still also.
Someday I hope to rewrite the client uploader in Electron.