Well, including both chips would increase the cost for everyone, and Smartthings is definitely trying to compete on the basis of price against controllers like Fibaro. There are also engineering issues in trying to build a board that would hold both, things just get complicated. There’s never enough room on these boards as it is. But you can talk to the company and ask.
Since they are now offering the hub model with the European frequency, currently being sold in the UK, I suspect that’s the direction they intend to go in the future. Just different models for different frequencies.
Z wave is not more popular than zigbee in terms of total number of devices. And it really depends on the device class. Many security systems and high-end automation systems like control 4 use zigbee. And of course in lighting, Phillips Hues have set the standard for the use of zigbee for smart bulbs. Battery-operated sensors can also be somewhat smaller with better battery life if they’re zigbee, so you’ll find a lot of choices there, including the ones sold on the smartthings site.
Z wave has been more popular for the low-end do it yourself market for complex devices that will interact with other devices. Everything from wall switches to pool controllers. The reason is probably that Zwave enforces interoperability more than zigbee, so devices from different manufacturers typically work together better than zigbee devices.
That means that customer support costs are probably lower for companies that are manufacturing Z wave if the device has to work with devices from other manufacturers.
The other issue has been interference with Wi-Fi. Zigbee and Wi-Fi use overlapping frequencies. Zwave does not. While it’s not that common, it is possible for a zigbee device to run into interference from a Wi-Fi network in the home, again raising the customer support costs per manufacturers offering those devices to do it yourself customers. Professional installers will have the tools and expertise to work around this problem. But note that light switches are probably the least easy thing to relocate in an existing home, which may well be another reason why Z wave has been more popular for that device class.
zigbee intends to address the interoperability issue with their next generation, zigbee 3.0. We’ll just have to see how that goes. Some of the security companies like adding their own layer of proprietary encryption, they feel it makes their systems less hackable. So they don’t really want their sensors to easily link into systems from other manufacturers.
The popularity of Hue is also pushing the development of more zigbee light switches, And they’re much easier to find than even a year ago.
You can certainly build an all zigbee system if you want to, some community members have done so.
One of the real advantages of a SmartThings installation is that you can choose a device based on the features of that device, and not worry so much about the protocol. 
