Repatriating cash earned outside the U.S. involves a hefty tax (afraid I don't recall the number). Occasionally the government offers a tax holiday to encourage companies to do so.
So, rather than bring the money home at the high rate, Apple has been taking on debt for U.S. operations while waiting for (and perhaps lobbying for) another tax holiday.
And I hope you know that the phrase "repatriating" implies something false. The cash is not stuck in a box overseas. Even though Apple (and google and others) would transfer their profits to a company they set up overseas where there is a lower tax rate, that money ends up in banks and holdings in the US. The hitch, to Apple or Google, is those monies have restrictions on its use, eg, they can't use that pile of cash to build a factory or pay dividends. But what the can do is use that money as collateral for a nearly equivalent amount as a loan which is not so encumbered. It just becomes a game for them to avoid paying most taxes until they are able to get an administration / congress which gives them a tax holiday so they can convert all that deferred money back into normal cash holdings at a much lower tax rate.