Why can Wordpress ship with latest version of jQuery but Drupal cant?
My understanding is that the version of jQuery gets locked down with the version of Drupal during development to ensure stability. It is all part of the testing process. The Drupal developers fully test the system before releasing a new major version of the CMS, including the JavaScript functionality. The incremental changes that go on throughout the year do not provide the time to test at this level so the jQuery version stays the same.

I am not very familiar with Wordpress, but my understanding is that they make considerably more regular major upgrades to the versions of their CMS. So jQuery gets updated more frequently. However, from what I've read little testing takes place.

While getting stuck with an old version of jQuery can be frustrating, I'd rather go for a stable well tested system.