The simple fact is that political Washington really can’t pay attention to two big stories at once. Or, more accurately, it can’t fight two battles simultaneously. It can’t walk and chew gum.
As soon as the IRS story broke 14 days ago, it became clear that the conservative energy in (and out) of Congress was going to be dedicated to getting to the bottom — or maybe the middle — of what happened. (Remember that it was conservative talk radio that doomed the last attempt to reform the immigration system in 2007.)
Congress, at least in its modern incarnation, tends to act like a moth drawn to a flame. It is a reactive institution — by and large taking action (or giving off the appearance of taking action) on issues that the public seems concerned about. Members of Congress pay attention to what they think their constituents want them to pay attention to — because they believe, probably rightly, that appearing to do the will of the American public (even if that will is somewhat scattershot) is the best course to re-election.
And, if Congress was focused on the IRS, it wasn’t focused on immigration. That meant questions about border security and, especially, the inclusion of a path to citizenship in the legislation didn’t engender the fight they might have otherwise.