Because it was an unfair move by the UK.
Why should Iceland's citizens pay for the mistakes of a private bank? At the end of the day, all bank accounter holders are investors in a bank and risk failure of the bank in return for an interest paid for their deposit.
Iceland, in the end, got out of and recovered from the GFC much quicker than may other countries. Furthermore, they had the guts to properly manage the situation with appropriate regulation. Whereas in the US it appears ALL banker got away scot free from the crash and are behaving in exactly the same way they did prior to the GFC (with maybe a few risk algorithms tweaked to profit from the next GFC).
Well done Iceland.
PS - It turns out that YOU ended up paying for the lost bank deposits of your fellow citizens you invested in the Icelandic banks - how does it feel to be burdened with the debt of a private bank?