I think one factor that's been overlooked in the previous posts was the fact that the Iroquois saw war differently than European powers did. I'm not an expert on Irqouois culture, and the research I've done for my TL has been on 16th and 17th century Iroquois culture rather than 18th cenutry Iroquois culuture, but I do think there were a number of cultural factors which led to the Iroquois (more accurately: some of the Iroquois) siding with the English rather than the French.
- to the Iroquois warfare was more an economic act than a political act. The goal of war was not to utterly defeat your opponent in order to impose your political agenda on them, but to raid your enemy for spoils. In the pre-colonial (and early colonial) period, spoils were usually in the form of captives used to increase the Iroquois population, but by 1700 access to beaver pelts had become more important than captives. Thus, the Iroquois were interested in fighting whichever power controlled beaver hunting grounds that they coveted. This was the French.
- the Irqouois-English alliance (and the Iroquois-Dutch alliance before that) had everything to do with the position of the various trade routes from the coast to the interior. The Iroqouis were very well positioned to bring furs from the interior to trade posts on the Hudson, Delaware, and Susquahenna Rivers, which is why they made natural allies with the English. The English needed the Iroquois as a middleman to obtain furs from the interior, as the Iroquois were positioned on top off all the important trade routes between the English colonies and the interior. The French, on the other hand, could and did easily bypass the Iroquois by using the Ottawa River-Lake Nipissing trade route. A French victory would mean that the Iroquois would be left without a trading partner, as, to the French, they were simply an unnecessary middleman.
- the French had offended the Iroquois a number of times with their missionary activity in the 17th century. I'm not sure how relevant this still was by the 18th century, but, to a large extent, the Iroquois didn't get along with the French BECAUSE they were less willing to convert to Catholicism, then, say, the Hurons were.
- the English continually made promises to reserve land for the Iroquois by limiting expansion of their settler colonies. See the Royal Proclamation of 1763. The Iroquois were more or less convinced by this promise, and didn't forsee the American Revolution being fought over this and other issues. Thus, while encroachment of settlers was a problem, it was seen as a necessary sacrifice to make in order to benefit from the enormous prosperity the Iroquois were able to gain from the fur trade with the English.