Because that's how virtual-host-style access works. Please read the link I've shared above (twice). It explains this.
I don't think we can offer much advice on how to configure your storage device to behave like S3 here. A brief google for "ECS S3 virtual-host access" throws up some links that may be helpful. If that doesn't help, I think you will need to ask for support from the supplier of your storage device.