Droprate sounds right. I'm not sure bad luck applies beyond the first one.
my kill count is about 20/30 and i have 1 loom, so youre not that unlucky
you dont just get them handed to you, droprate is not 100%
You have far more heirlooms than I do. I don't know what the going rate is, but being unlucky would mean that you hadn't got any heirlooms yet; you're nearly into double figures for goodness sake.
40+ kills over different characters and different difficulties.. 0 looms. Yes, that's right, 0. None. At all. Stop whining.