Why can’t you do java script for iPads?

It all depends on the PDF viewer you're using. Readdle's PDF Expert supports much more than Adobe Reader. We've been trying for quite some time to get Adobe to add support for more JavaScript features, but they don't seem interested in doing it.

Adobe did. Unfortunately, they only did about 1% of the work.

JavaScript for Acrobat Reader Mobile API Reference (iOS)

The failure to offer feature parity is disappointing.