We’ve removed jQuery as a frontend dependency for the Material Design and the Polymer Single Page Application Starter Themes.
Please note that jQuery is still needed on the WordPress dashboard and will be loaded there!
jQuery was very useful a while ago but browser support for modern web standards has improved a lot and most JS frameworks started to drop any external dependencies in favor of plain/vanilla JS.
Reducing the loading time of web applications and minimizing HTTP requests is one of the main goals of many developers. And since jQuery is an external library which needs to be downloaded first, avoiding it (if possible) makes a lot of sense.