I realize that this is an extremely broad question. But it is my understanding that the industrial revolution marked the biggest transition in human societies since the invention of agriculture, so it seems like a very important question. Why did it happen when it did?
Ignoring the time period is a bit off, this is a very complicated question, one to which we aren't quite sure on. It might be better to ask "Why did the industrial revolution happen at all?". It appears that it could only occur, or at least was only likely to occur, with a large number of conditions that didn't come about till the late 17 to mid 18 hundreds.
Some conditions that have been deemed important are:
there are others, but the whole thing is a wholly mess that probably won't be finished untill after we're all dead