I think NetSuite said it is because of uncertainty with browser support. While nearly all modern browsers do support many features of ES6 or later, SS 2.1 uses ES10+, so many features in 2.1 might not work across all supported browsers. It would be nice to have a limited subset of 2.1 for client scripts, or just trust customers to test things in the browsers they want to support, and not use features that don't work, or to use polyfills.