When we first evaluated options, zip.js only supported IE9+ (and IE6-8 were still relevant browsers) while JSZip supports IE6+. In fact, the Base64 text box was introduced in http://oss.sheetjs.com/js-xlsx/ specifically to test IE6-9 (which do not support the HTML5 File API)

I see. Inevitably its support for older browsers makes it slower, however.