Shouldn't be surprising that they wouldn't form a coalition with the Independence or Progress parties.
The Independence Party (Sjálfstæðisflokkurinn) is basically Icelandic Republicans. It's too good of an analogy not to make. If Republicans in the US like it, they like it. They're maybe not as hard on the social conservatism, but economic, yeah, they can party with the best of them in the US Republican Party. Anti-EU.
The Progress Party (Framsóknarflokkurinn)... this is a beast that you really aren't familiar with in the US. Sometimes they're referred to as right-populism, but really I think the best way to describe them is the "Idiot Party". Generally they do terrible in the polls right up until a couple weeks before the election, when they come out with some Big, Super Plan, which basically amounts to "We're going to give you tons of money, and you're never going to have to pay for it, like ,not EVERS!" There's so little time before the election that idiots get enough time to hear about it but not enough time to hear about how utterly terrible it is, and Framsóknarflokkurinn surges in the polls... then their support quickly collapses after the election, but who cares about it then? They're in government and can enrich themselves and their friends to their heart's content. Anti-EU.
The Pirates on the other hand could easily form a coalition with a number of other parties:
Samfylkingin (not sure what the English translation for them usually is)... as much as Sjálfstæðisflokkurinn is Iceland's Republicans, these people are really Iceland's democrats: left-center pragmatists. But then again, the left in Iceland is further left than the US. Pro-EU.
Left Greens (Vinstri Grænir): Yeah, there's also a Right Greens, but they're a small party, no need to talk about them. The Left Greens are a traditional Green Party... Left-Idealists. Anti-EU.
Bright Future (Björt Framtíð): Relatively new party. They're another leftist party, with some stances matching with Samfylkingin but others matching the Left Greens. Pro-EU.
The Pirates have no pro or anti EU stance, except that people should get to vote on it. They're very much not happy with our current government's promise breaking and lawbreaking on this front. But the membership is mixed on how they'd actually vote - they just want to get a vote.
I think the Pirates would form a great part of any potential leftist coalition. They have a lot of policy blind spots where they try to avoid taking stances, but they're very hardcore on certain issues that really need an advocate.