This correlation is not always true, an in emerging markets, this correlation is almost always wrong. Access to Smart phones unfortunately doesn't always mean the availability of banking infrastructure OR the presence of a cashless economy . For example -

- Android phones are dominant in this market, since local manufacturers are able to sell cheap phones without incurring any software development cost.

- Telecommunication sector is heavily privatized in these markets & generally have multiple players offering low cost "pre-paid" services. Recharge stations and mobile stores are pretty ubiquitous - small mom & pop shops double up as resellers of these services , providing last mile coverage.

- On the contrary, Banking infrastructure is so heavily state run. And they don't have the same level of 'ease of access'. So, a lot of people working in the "unorganized sector"(Note: This is an official terminology that governments recognize and use) don't have formal accounts in financial institutions.

- Even if they did, a good number of banks run decade old tools and softwares. Making it all the more difficult to do mobile & online banking .

- And also, most of these economies are not cashless. Culturally and for other reasons,there is more trust on "Paper Cash".

- Finally , just because a person owns a cheap smart phone & ,one cannot assume banking literacy.

Too naive an assumption. I know of certain online payment platforms that have to blacklist certain regions of the world because of fraud detection and the like..