I suspect they have some numbers on how much the ipad cellular is sold and how often that feature is used and they may be using that to determine whether it's worth it.

But now that they develop the chip in house, I don't see why they wouldn't include it. In fact, given they put an Apple Watch chip and extra display in their macbooks for a few years, I don't see why they haven't done it years ago.

edit: I googled, apparently Qualcomm's licensing fees are a percentage of the whole device the chipset is installed in. I suppose it's less of a factor now that some iPhones are more expensive than Macbooks though.