Not necessarily cheaper people, but people who don't require long term investment.
Like many employers these days, IBM is thrashing. They jump from one trend to another, hoping to catch a hot one but never quite succeeding. The ideal IBM employee these days is a contractor, which is why you see so many of them around (or at least you did when I was there). They don't require costly training, they aren't paid benefits, and you can get rid of them when the trend flames out.
If you're looking for work at IBM, then you should expect nothing more than gig-based temp work...because that's what IBM is all about these days. Great when the money is there, but when the money runs out so do you.