By the very same logic Microsoft should push as strong as Apple because it makes even more business sense for them. They don't sell hardware. Their profit comes entirely in software. Do we see the same behavior? Hell no.
And risk alienating large businesses who want business critical legacy apps supported? I don't think so. It wouldn't make sense for Microsoft.
Correct me if I'm wrong here, because I don't have the figures to back this up, but I'd imagine most of Apple's revenue from Macs comes from the home, rather than a business environment. It's easier to convince a home user to upgrade to the latest and shiniest version than it is to convince a cautious and conservative IT department.