I Agree with everything said above except that I think you will find there are many fewer zigbee devices available in the U.K. that would be compatible with SmartThings than you would find in the US. This is a common complaint in the UK section of this forum.
While it is true that there is only one zigbee frequency, the zigbee standard allows for the use of multiple “profiles” and these are not all inter-operable. They don’t even use the same addressing structure.
SmartThings uses the “zigbee home automation” profile (ZHA 1.2). These devices much less popular in the EU than zwave.
In addition, different countries set different maximum ceilings on the transmission power of zigbee devices. The level allowed in the EU is about half of that allowed in the US, which means that many US zigbee devices are illegal to operate in the U.K.
There are some zigbee devices which use the ZHA profile and are available in the UK, in particular the Orvibo brand, but in general you have many more choices in each device class in the UK if you look for zwave rather than zigbee.
The main reason may be that Wi-Fi and zigbee operate in the same band and WiFi can drown out zigbee. This makes device placement much trickier for DIY installs.
Z wave operates in a completely different band and does not suffer from Wi-Fi interference, so it is generally easier for do it yourself installations. This probably contributes to the popularity of the protocol.