Wasn't the New Amsterdam wall made of wood? They wouldn't need cannons, just soldiers.
And yet the Confederacy doesn't have any sort of regular army, what warriors they get aren't going to be a force well versed or prepaired to capture and occupy a small European City, wood walls or no. Plus no force they can raise is going to be that big. And of course, how can they even afford to leave that force in New Amsterdam after they supposedly take it? Thats not how First Nations fought wars. Warriors came and went, often as they pleased, and would continue to do that all the way to the War of 1812 in the Northeast.
As mentioned the Mohawk territory was at the other end of the Hudson River, its not the easiest commute to make, especially when many of the Algonquin tribes in the area are blood enemies of the Iroquois.
And lastly, Iroquois dominance was in a large part due to the fact that they were able to trade with the Dutch, and later the English. Their triumph in the Beaver Wars would be impossible without the arms, powder, ammunition and supplies they traded the Dutch for. If they're going to war with the Dutch they're attacking the hand that feeds them. If they take the city how do they even maintain their strength to outlast the seige it would take and to hold it against any takers?