The cash is collecting dust abroad, and large tech companies are waiting for a tax holiday, for which there is precedent [0].
There are a number of fascinating reasons why companies may hold cash (e.g., it's been suggested that companies with CEOs that grew up during the Great Depression hold (held) more cash than those that didn't). [1]
That said, it does appear that taxes seem to be one of the more significant (if not the most significant) reason in our current economic climate.
[0] https://en.wikipedia.org/wiki/Repatriation_tax_holiday [1] http://www.rhsmith.umd.edu/files/Documents/Departments/Finan...