Requires provisioning from the mobile network telecom provider. In Nigeria, they are adding USSD support for the e-Naira (CBDC), so that it can be used from feature phones.

The problem is, in many developing countries, the government either owns the mobile network companies (in full, or substantially), or controls them via either regulation or edict. And if the government in a country is hostile to bitcoin, they certainly won't be permitting provisioning of USSD for a bitcoin-related endeavor.

There's another approach, known as Sim Toolkit (STK). Same issue, ... the SSL certificate for the service must come from the network telecom provider, which they likely won't do for bitcoin-related endeavors.

So that's the choke point.