A couple reason legacy PCI slots are still included, even on mATX boards. First, these are H170 boards. They can't split CPU PCIe lanes so they can't take a second GPU ( CFX technically is supported by a x4 link through the chipset, but doing so is not advised ). So, one x16 slot is enough there.
Second, that these are H170 boards means these are the lower end fare that most casual computers and office spaces will upgrade their hardware to. A lot of them still use older PCI cards, particularly wireless networking cards. So including, a legacy PCI slot means they can continue to use their older cards that are likely still performing well enough for them.
Finally, what kind of slot would you prefer to see? Most people don't even use one PCIe x1 slot, so what would they need two there for? With quality networking, audio, and RAID controllers integrated into the mboard, there's not a lot of reason for add-on cards.
While the question is valid, it may be more important to think of what feature should be there instead, or if adding that one slot is adding unnecessary cost to the board.