Ambers carefully considers it:
Here is a thought experiment: Going over the fiscal cliff would have damaged the economy and thrown it back into a recession. Defaulting on the national debt would probably send the economy into a depression. Why would the Republicans, who, after all, were reasonable enough to vote on a tough deal to avert the fiscal cliff, refuse to make a deal on legislation to avert the debt ceiling default, which would imperil the country and anger their donor base even more?
It may be wiser for Republicans to use all of their leverage to insist on deep cuts during the sequester fight without upsetting the financial markets by holding the debt ceiling over the president's head.
Ezra Klein's view:
In the end, [Republicans] weren’t even willing to go over the fiscal cliff. The debt ceiling would do far more damage to the economy than the fiscal cliff, and Republicans would receive far more of the blame. After all, many thought President Obama did want to go over the fiscal cliff as going over raised taxes, and so it was possible Republicans could’ve portrayed the breakdown in negotiations as a Democratic strategy. No one thinks that the White House wants to breach the debt ceiling, and so Republicans will take all the blame.