I've always wondered the same thing as well. It's a great destination and the Hilton family has such great coverage through TX, it would only make sense.
The nearest property is an Embassy Suite in McAllen I believe. About an hour drive to the island. It's fairly nice as any Embassy Suites always is.
In my trips to the island, I have always stayed at the Radisson. It's a nice property with a pretty good staff.