A key requirement for eUICC’s to operate in a device ( see Annex G of SGP.02 ) is that the device supports Bearer Independent Protocol ( BIP ). This protocol exists only between the device and the eUICC and enables an eUICC to request the device to set up a data session with the SM server. This provides a much faster method of access to the eUICC than SMS. A good explanation of BIP can be found here.
Unfortunately not many devices in the field support BIP. Even though new devices are being designed to incorporate support for BIP, this has severely restricted the adoption of eUICCs