The short answer is because it exceeds 10kg in weight (due to it's unique recline feature), which is the maximum weight a car seat can be to use ISOFix installation. Therefore the seat-belt installation method is used.
But it's not all bad, especially once you understand the purpose of the ISOFix system.
There's a common misconception that Car Seats with ISOFix are automatically safer and more modern than non-ISOFix Car seats. But that's not necessarily the case.
Here's a few FAQ's to help break down ISOFix and it's role in child safety:
What is ISOFix?
ISOFix is a system designed to reduce the chances of incorrectly installing a car seat. It comprises 2 anchorage points on a vehicle seat which correspond with attachment points on a baby Car Seat. The attachment points on the baby Car Seat click onto the attachment points on the vehicle seat. Because the ISOFix system ‘clicks’ onto the vehicle seat rather than relying on the car seat belt to be adjusted to the correct tension, the chances of incorrect installation are greatly reduced. Obviously, a correctly installed car seat is much safer than an incorrectly installed car seat.
See a more detailed overview of ISOFix here
What are the benefits of ISOFix?
ISOFix gives you peace of mind that your car seat is correctly installed, and is usually faster and more convenient when changing Car Seats between multiple vehicles.
For most Car Seats (other than Capsules), the child restraint is 1 single item. So when moving it between cars the whole seat must be completely removed and reinstalled each time.
However, Capsules have 2 parts: the Base, and the Capsule itself. Of these, only the Base is installed into the vehicle using the Seat belt/ISOFix. The Capsule section is lifted in and out of the car to attach to the pram or to carry, with your baby inside. The Base remains installed in your vehicle.
Is ISOFix installation safer than Seat-belt installation?
ISOFix is considered safer only due to the reduced chance of incorrect installation. Other than this the ISOFix system itself does not increase the level of protection in an accident - in fact crash testing shows that seat-belt installation is often safer than ISOFix installation.
Why isn’t the CYBEX Cloud Q available with ISOFix-compatible base?
This is due the weight of the Cloud Q child restraint exceeding 10kg, which is the maximum limit car seats for ISOFix as specified in the Australian Standard. Therefore the seat-belt installation method is used.
Does this mean the CYBEX Cloud Q isn’t as safe as other car seats in the market?
Not at all! In fact the CYBEX Cloud Q has achieved the highest rating of all Capsules in the market. See more here.
Compare the safety ratings of all Capsules in this category here
So how can I use the CYBEX Cloud Q and ensure it is fitted correctly?
For peace of mind, we recommend having your Cloud Q professionally installed. If you plan to use the car seat regularly in more than one car, we recommend to purchase an Additional Base to be (professionally) installed into the second car. This way you can click the Cloud Q in and out of either car as you wish, without having to remove and reinstall the Base.